Admittedly I dont know a whole lot about laptops but is a 100gb HD standard for them? Seems kinda low...
I think it depends on where you are in the product range. For "budget" machines (under ~US$1200), 40GB is probaby "standard." Indeed; for this particular Alienware model, which is
not a budget machine, the base hdd is 60 GB, 5400 RPM. But unless you're doing video editing, have a large number of hi-rez photos (or a massive collection of jpegs), or are hoarding MP3s on your laptop, 100GB should be more than plenty. I have three 36-GB hard drives in my desktop machine and I've got a total of 54 GB free—and I probably have more stuff installed than most people here (but the only MP3s I have are the ones I use for the game).
Larger-capacity drives can have a bit of a performance advantage due to the data being packed more tightly, but for most people the rotational speed will be a more important performance parameter. "Standard" laptop drives are still 4200 RPM, the midrange drives are 5400 RPM (the minimum you should accept, really), and high-end drives are 7200 RPM. By comparison, the standard for IDE/ATA and SATA desktop drives is 7200 RPM and the high-end is 10K; for SCSI drives, 10K is now standard and 15K is the high end.
If this machine really is new (and not a counterfeit), then it is certainly a good deal. In this configuration, the same machine direct from Alienware would be about $2800. The current model has a 17-inch display and nVidia graphics, and the 100-GB hard-drive option spins at 7200 RPM. Interestingly, Alienware declines to include the weight in the specs, but expect it to be heavy. Also expect it to be hot and loud (from the CPU fan or fans) because it uses a desktop CPU and not a low-power-consumption Pentium M (Centrino)—it will probably be too hot to actually use on your lap.
You will probably want to verify the components once you have it in your posession. You can see most of this in Windows Device Manager, but a free utility like
CPU-Z will show you the exact CPU model and RAM modules.