How can I deliberately slow Windows?

We use a 'crippled' server we call doofus for load testing. it is an old P3/500 box with limited RAM.

Another option is setting up a VM with very limited resources.

Use powercfg.exe, to force the computer to a power plan you've created that locks the CPU into a lower frequency to conserve power. What states are available depend on your platform (most desktops only have a couple.)