I want to stress-test my application this way, because it seems to be failing in some very old client machines.
At first I read a bit about QEmu and thought about hardware emulation, but it seems a long shot. I asked at superuser, but didn't get much feedback (yet).
So I'm turning to you guys... How do you this kind of testing?
I'm not sure about slowing a CPU but if you use a virtual machine, like VMWare, you can control how much RAM is actually used. I run it on a MBP at home with 8GB and my WinXP VM is capped at 1.5 GB RAM.
EDIT: I just checked my version of VMWare and I can control the number of cores it can use. It's definitely not the same as a slower CPU but it might highlight some issues for you.
Since it's not entirely clear that your app is failing because of the old hardware or the old OS, a VM client should allow you to test various versions of OSes rather quickly. It came in handy for me a few years back when I was trying to get a .Net 2.0 app to run on Win98 (it can be done though I don't remember how I got it working...).