More Awesome Than You!

I am so upset about my computer right now. I have no idea why but it keeps shutting off.
I know it's not a "Thermal heating event" or a virus as I ran a full scan last night. It only happens when I am playing my game though.
I cut down my downloads from 8.5gb to 6.77gb. >.>
I also cut back on the hacks and hacked objects I have.
I can play for a little while but then after a while my entire system just shuts off.  No blue screen or anything. When I turn it back on it comes on fine as if I shut it down myself.
No wrongful shutdown messages.
I have no idea how to mimic the event so I am not sure how I can test it to see if it is a download or hack.
Any troubleshooting ideas or advice would be much appreciated.
My pc specs are as follows.

Windows Vista Home Premium Service Pack 2

AMD Athlon 64x2 dual core processor 5600+ 2.8GHz
3.00GB RAM
32-bit OS
Graphics Card: ATI Radeon HD 2600 PRO
Mem: 1523 MB

I have all the expansions and stuff packs.
I don't know why it's acting up all of a sudden. I've had this pc since August and I didn't have any issues with it. I love my pc.
Please help... :(

Here are the thoughts that came to my mind:

1)  The motherboard is going bad.  (We've had this happen to us.  Computer would shut down more and more, until eventually it refused to come back on.)
2)  Processor is going faulty.  (Probably not, but possible.)
3)  Memory is going bad.  (See #2)
4)  Windows Vista.  (I just hate Vista, although Windows 7 is fantastic.)

It's probably the fact that you're incredibly stupid and fail at life.

Aww.. shit... :(
That totally sucks.
What do you suggest I do? Should I get a new motherboard or memory or should I wait it out?
I really cannot be PCless.
I don't even really know how to buy a new motherboard. :/
Memory I can deal with but the motherboard is just plain scary...

I suggest becoming An Hero.

Before you buy something new, you really need to determine which part is failing.

Of course, without either buying a new computer, or having lots of old computers lying around, there is nothing to test the equipment against. This is why you have to keep all your old computer parts.

Before trying anything drastic, try some simple troubleshooting first.

Does your computer shut down when doing other tasks besides playing Sims?

If No:
Backup up your downloads, custom configs, Neighbourhoods, etc... and then delete the entire Sims 2 folder in your documents to reset the game. Play for a while with the game in its vanilla state - no downloads at all. If all goes well, you will need to troubleshoot further. Something in the game, itself, may be causing some sort of memory issue. Also, try playing with the video settings in the game.

If Yes:
You have something wrong with either your OS, or your PC. Scans don't always catch problems, and a re-format is less drastic than replacing parts, or buying a whole new PC, even though it's more of a last resort than anything else.

Basically, think in simplest terms first. If the problem only exists with your game - that's where you start. If it exists with your entire OS, start there, etc…

Any condition that causes a computer to outright shutdown is always some kind of driver or hardware issue. No application should be capable of causing a computer to shutdown without a corresponding hardware/driver problem such as overheating.

I had a problem like this before, and the solution turned out to be very simple. Apparently my cat had decided there wasn't enough room in front of my tower to lay, and gradually pushed it back until the wires were smashed up against the wall. When I tried to play any  it would shut down at random intervals, but when I noticed and pulled the tower away enough the problem stopped. Do a point-by-point check of all the simple possible problems before doing anything drastic like replacing parts.

This is also why you should check things like fans for too much dust or hair and to make sure that they are actually turning and cooling like they are supposed to. As Pescado said, overheating can cause this problem as well. Does it shut down with other games? Have you even tried other games? And I don't mean dinky games, I mean something that requires your graphics card to do a little work. Have you tried uninstalling your graphics drivers and installing new ones then repeating the trouble shooting?

You're right, well, partially - there are programs that can shut down the PC, be it by fault or on purpose, which is clearly not the case here though.  :P

I was half asleep and not thinking clearly, my bad.

@OP:
Are you using a laptop or a tower? Sometimes the PC randomly shutting down is as simple as it overheating because it needs to be cleaned or the vents are blocked - needing to be cleaned is more common for laptops, but I wouldn't overlook the possibility for a tower. You may have something blocking your vents, like having your tower in a desk cubby. Try moving it to a cooler and more open spot if that's the case. Overheating can be caused by a number of issues. So, I still recommend trying the simplest solutions first.

The next place I'd look would be drivers if the above doesn't solve the problem, or appear to be a problem, and from there, moving onto hardware troubleshooting to replacements, or to more drastic measures like reformatting or buying a new PC if nothing else will solve the problem.

It could be such a large number of things that it's hard to troubleshoot without knowing more about your system.

ETA: because I'm so slow to post things.
Follow morriganrant's advice too.

You might check the power supply, too.  If it's loose or being bumped or jiggled that could be causing the computer to turn off (assuming a desktop). 

First of all thanks for all your advice and tips.
I did move my tower away from the wall and I also determined that it was getting a little warm where I have it. My father (computer geek extraodianaire) told me it might be my power supply or my motherboard. But it's been days and nothing has happened. I gave away all my old pc parts (hoarder ) to the "computer guy" down the block. (stupid...)
I am going to try a vanilla game and see if it shuts down. As I recall the last time it shut down I had just set my smooth edges slider all the way up cause I had a "..... moment".
The other time it shut down was when I was quickly flipping through my downloads. I suspected my videocard but meh.
I clean my pc regularly but it's been about  a month so I think I'll look into it. The spot it's in is not very good I figure so I am moving it.
Hopefully it will work out.
If not I will have to wait and let my pops take a look when he gets here next month. Til then I will keep things cool and slow.

*Edit*
I have determined, after trying to run bodyshop and second life. That it is not just my game that is crashing as these two crashed my pc as well, after I cleaned the dust out.
I also smelled something kinda burny.I don't know what it could be though. I am going to "borrow" my husbands video card to test if that is my problem. All of my fans are working fine. I have three, one for the power supply, one in the case for general filtering I guess and one on a heatsink.
Any ideas what the burning smell could be?
Could it be my motherboard or my vid card?

Re-edit*
Turns out it was my vid card. Got a new one and sending the other one to be repaired. The fan failed and made it overheat causing the system to shutdown as a failsafe. :)
Thanks for all of your help. :D