Well, first the bad news: XP needs more resources than ME. The most important of these is memory. Although it will run on 256mb RAM, it's better to have 512mb. If you've got an old computer without room for sufficient memory, I'd start thinking about a new one.
That said, it's possible to run XP on something like a 350mhz processor and 256mb RAM.
The easiest way to install XP would be to buy an upgrade disk. But you might prefer to simply have the regular XP disk because when you re-install, you won't be upgrading and if you use the upgrade, you'll have to re-insert your ME disk or equivalent each time you re-install.
So, if you think your computer can handle it, get an XP disk and stick it in the drive. You'll need to set your BIOS to start from the CD, and as soon as installed, set it back to boot from whatever it's set at now. Usually, it's A: first and then C:.
You'll want to back up all your data first, which you should have been doing all along. Banking info, favorites, pictures, documents, etc. (If you upgrade, this info should be saved, but in the event of a problem, you could lose everything.)
Do you know how to enter your BIOS and change the boot order? That's the first step - usually by pressing the Delete key as the computer is starting, then using your arrow keys to move through the tabs and menu choices. Once in there, you'll see instructions to move around. Then set the boot order to start from the CD-Rom first, hard drive second. Change it back once installed.
When you open the XP package, pay close attention to the PRODUCT KEY. This is important. It's in sticker form and if you have a shrink-wrapped version, it's easy to lose or damage it. This sticker should be attached to your case, either on the side or on the back to make it easy to find.
Then start the computer with the XP disk in the drive and basically just follow directions. You may want to read up on some guides first so that you have some idea what to expect. You should make sure you have, at the very least, a good antivirus to install before you connect to the internet. I recommend the free version of AVG from grisoft.com.
Most versions available today have SP2 already installed, so you can skip anything about SP1 or SP2.
Here are some sites where you can read more about installing XP:Michael StevensPaul ThurrotEldergeekTweak 3D
I usually use OEM disks to install XP because they're cheaper. However, in order to buy one, you have to buy hardware with it, such as a motherboard and processor. You can search sites such as Tigerdirect.com and Newegg.com for OEM versions, best for when you're building a new computer. Win XP Pro is preferable if you're planning on networking computers, otherwise, XP Home is probably sufficient for the average user.