Sorry for the delay in replying. I've been too busy at work to check AB for a few days.
If you can do a fresh installation then an 'on-top' installation should be really easy for you. Having wriiten that, I ought to advise you that I use Windows Millenium and not XP. (I use XP at work and I really hate it!!). As far as I know, re-installing XP should be no harder than re-installing ME unless you come across problems because you've added new hardware. (From reading posts here - and from talking to XP users - I understand that XP is configured so that you can't install one copy onto two computers. Problems can arise if you try to re-install XP on a substantially modified computer because the software doesn't recognise the PC as the original computer. This applies to both 'fresh' and 'on-top' installations. But, as I say, I'm no expert on XP!).
Disclaimers out of the way, here's how to do an on-top reinstallation:
Boot from a floppy. Select 'boot with CD support'. The on-screen information will tell you which drive letter is assigned to your CD drive. Type ' E: '(or the appropriate letter) to switch to your CD drive. Hit 'return'. Insert Windows disk. Type 'setup'. Hit return. You'll probably get a prompt asking if you want to try to repair Windows or to re-install it. You can try the 'repair' option if you like but it's never worked for me! (i.e. I recommend the 'reinstall' option). Put the kettle on!
Chris