Helping people with computers... one answer at a time.
After a series of operating system changes, the your options for a true "restore" are limited, and usually involve a reinstall instead.
I have my nephews my old desktop computer, which had Windows XP on it. However, someone put a Windows 2000 disk in (just see what it was - I guess) and now the OS is Windows ME. Is there any way to restore that computer back to XP?
I say that because there's no way that putting a Windows 2000 disk into a machine is going to result in that machine running Windows Me. And installing an operating system - ME, 2000 or even XP - doesn't just happen by accident; it requires interaction with someone who's explicitly answering questions about how the operating should be configured.
I'm guessing that a lot more has happened to that machine than your question would seem to indicate.
"Restoring" to Windows XP could mean several things.
If you're just looking to get XP onto the machine and there's no data to be preserved, then the best solution is to get the original Windows XP installation CD that hopefully came with the machine and simply reinstall Windows XP from scratch. If there really is no data to be preserved, then I'd strongly recommend having Windows XP Setup reformat the hard drive in the process so as to have a completely clean machine running Windows XP.
I have an overview of setting up a machine simply in this previous article: How do I set up my computer - simply?.
If you're wanting to restore the machine to it's previous state, which means more than just the operating system but includes applications and data as well, things are much more difficult.
You probably don't have it, but I'll say it anyway to point out how useful it would have been: if you'd been using a backup program to backup the entire machine, you could simply restore to the most recent backup made prior to losing Windows XP. Most backup programs actually make this quite easy albeit a little time consuming. And the net result is that you would have had the machine back exactly as it was at the time of the backup.
Given how invasive a new operating system installation is, your chances of actually getting back to what you had before by "undoing" the operating system installs is pretty much zero. Honestly, an installation of Windows XP from scratch or a restore from backup are by far your best bets.
If you can't take those paths then here are two more straws to grasp at:
Upgrade Install - rather than a full format-the-hard-disk install, use your Windows XP CD to install and "upgrade" the machine from Windows Me. This should preserve most of the machine's settings and your data. But I'd be sure to so a back up first just in case.
Use Dual Boot - you didn't mention it, so I'll assume it's not there. There's a small possibility that your Windows XP installation is still there. If when you boot you get the option to choose your operating system and Windows XP is listed, then you may be able to simply remove the other choices. Normally I'd then also tell you that you could go and delete the folders or partitions that might represent the other operating system(s), but in all honesty given what your system has been through I'd be too concerned about accidentally erasing something important.
Even if either of these approaches succeed, I still believe strongly that you should then backup any data you want to preserve and reinstall Windows XP from scratch. It'll give you the cleanest, most stable result.