Tools for Microsoft Windows
Consume.exe from the Windows Server 2003 Resource Kit Tools can consume various resources: physical memory, CPU time, page file, disk space and even the kernel pool. Although for Win2003, it should install into WinXP fine (Win2000 compatibility is unknown). The only downside is you can't specify how much of the resource to use, only which one to consume. Also be warned: always remember the -time option, or you may be hitting that physical reset button sooner than you would have liked!