That's usually in the BIOS. I don't know about other PC makers, but if you're using a dell try this:
Reboot system and enter the BIOS Setup by pressing F2.
Turn on Caps Lock, Scroll Lock and Num Lock.
Press ALT + E then ALT + F (should hear beep) then ALT + B
OS Install Mode will limit you to 256 MB RAM or whatever the install mode cap is.