You have two choices. I'll offer them in the order I'd use.
1. BURNMEMORY. Not literally but here's the article about it and editing BOOT.INI -> http://support.microsoft.com/kb/833721
2. Virtual PC. Google that but it's only for the exit of last resort.
My youngest is very interested in animation and editing. I gave him my old copy of Adobe Premiere 5. It ran painfully slowly so we upgraded his PC. I suspected that something as old as Premiere 5 wouldn't be happy under Vista, which is what the new Dell PC (Inspiron 530) came with, so I made his new machine dual boot and installed Premiere under XP (SP3), which is what I'd been using it under.
But Premiere can't copy with the 4 GB of memory in the Dell; it thinks there isn't enough memory! Is there any way I can reduce the amount of memory visible to Premiere without actually yanking out 2 GB?