32 bit OS's like XP limit each program to 2gb each add in that if you have 4gb installed Windows will only see and use anywere from 2.5 - 3.5 of the ram plus any memory your video card has will decrease the amount of usable ram for the OS and applications.

I'm soo glad I went from XP to W7, 64bit is such a release from the contraints of 32 bit OS's. Now that I have 8gb, the only programs I've actually run out of memory with are Vue8 and Vegas Pro. Since then I've lowered Vegas' memory usage to 2gb instead of 7gb and it hasn't crashed since.