Force Application to use single core on dual core machine

I had a Dual Core system a year ago running with Windows XP. I wanted to to make it run on Single Core which I done by running a command prompt. Now I have a new computer with same XP and Dual Core and want to do the same. But the problem is I’m not able to recall that command.

So anyone of you know what is that particular command to run on single core (either "core 0" or "core 1")? Please let me know soon.

Well i haven't used such command personally so i cannot be sure but as i wanted to help you out and also to gain knowledge for myself, i did searches on Google and from what i found i think you were probably using something like : start /REALTIME app.exe