The VM (read http://en.wikipedia.org/wiki/Virtual_machine ) in Windows NT/2000/XP/2003 is a 32 bit OS. This means the largest memory space available at any one time is 2^32 or 4GB. While this seemed stratospheric years ago and mere mortals could not afford GB of RAM this has changed in the past 2 years. Today owners can order a laptop with 2 or 4GB of RAM and promptly discover the stock 32-bit OSes have some interesting nuances.

XP 64 is a 64-bit VM with "the theoretical memory limit a 64-bit computer can address is about 18 billion GB (18 exabytes), Windows XP x64 is currently limited to 128 GB (237 bytes) of physical memory and 16 TB (244 bytes) of virtual memory."

More at http://en.wikipedia.org/wiki/Windows_XP_Professional_x64_Edition

Bob