XP uses a lot of RAM. Right now, I just have Outlook and IE open and the RAM usage is 323 MB on my laptop.
You should purge your RAM before each new RAM preview in After Effects (“AE”): edit>purge>image caches. The RAM does not purge automatically when you make a change to the comp.
If that doesn’t work, get more RAM. In my opinion, you should have at least 2 GB to work comfortably in AE, but 1 GB should do in a pinch.
Steve