Preventing simple local DOS attacks

After years of Microsoft reengineering Windows with an emphasis on security,
I still see no way to give a user an account on my computer without also
giving him the ability to (accidentally or intentionally) perform very
effective DOS attacks by allocating gobs of memory, with the result that
switching users or performing any tasks under any other user accounts is so
dreadfully slow that I just give up and want to throw the computer across
the room because it's doing nothing but grinding away constantly on the swap
file.

So, in Vista, how do I set memory quotas for users, so that if their
programs try to suck up more memory than they're allowed, a UAC dialog pops
up asking for an administrator password?