With virtual machines, you can assign the amount of installed memory for it to use.

Example would be if the “Host” OS is XP, you can allot, say 256mb of installed memory to it, and all the rest to the “Quest” OS.

Having the virtual machine running, with no security programs, firewalls, updaters, internet, and especially all the MS services that can be turned off, more than compensates for that 256mb loss of memory.