First of all camstop, don't upgade
Let me clarify that
If you are going to change operating system from Windows 98SE to Windows XP,
don't use the upgrade facility built-in to XP, you'll only regret it later.
In-place upgrading can often cause instabilities, as you are often upgrading over existing problems (whether you know you have them or not).
The best way to install XP is a clean install. You will need to backup your data first (at least twice!) Then wipe the drive and install XP.
Another way, if you have the cash is to buy a new hard drive and install XP on that, leaving your original drive intact so that you can drag your data off it at your leisure.
There's probably loads of other things I've forgotten, but I'll let the others fill in the gaps