My experience... I installed Celebrations when it first came out and didn't notice anything all that new but from what i was reading it was mostly about parties and such so i didn't worry about it to much...
I assumed seasons would be too much for my computer so i never installed that... so i've got all ep's and sp's except seasons installed... I thought i'd check out H&M to see what it came with... i went through the same prob as reeseinpieces but did what katenigma suggest and that seemed to let it do a full install...
Tried to start the game with the H&M shortcut but that didn't work... so i used the pets shortcut instead... I got the CELEBRATIONS splash screen.... eewwww... thought i'd check out the clothes and found that now the celebrations stuff is enabled, but no H&M clothing...hhhmm wierd... i wanted to check out any new objects too but trying to enter any lot would crash me back to the desktop...
I finally uninstalled H&M and now i can enter lots but i've lost all the celebrations stuff too even though it's still installed...
I would be so pissed off if i actually paid for them. I think it's a way for EA to force you to install seasons... Evil EAxis