If you have an iPod disaster all may not be lost.  I hope my experiences are helpful to others.
They are marvelous things – until you drop them!  Mine is a 4th generation which had a 20Gb capacity. Recently I’ve had to replace a broken hard drive, and also the battery.  It is much cheaper to do it yourself rather than have Apple to repair it at great expense, and a better option than ditching it, even though it may be getting rather old.

After some Googling I found this useful site:-

http://www.ipoddoctor.co.uk/

which sells replacement batteries and has a neat video showing how they are opened:-

http://www.ipoddoctor.co.uk/4gmovie.php

Not having any instruments I actually opened mine with a kitchen knife!  However a thin piece of strong plastic like a guitar plectrum is ideal.  The hard drive is the same as those found in laptops.  I replaced mine with a MK4006GAH drive, thus doubling the capacity to 40Gb.

http://www.ultratec.co.uk/stocklists/SearchResultsPCB.asp?model=MK4006GAH

But you can even go up to 60Gb (MK6006GAH).  On plugging it back in to iTunes it re-formats it ready for re-loading.  It is a good idea to check the sides of the ipod base don’t need bending back in to position before closing it up! When the battery died it wasn't too difficult to change - using that trusty knife to carefully prize off the old battery.

The only problem was that a couple of weeks ago my desktop hard drive went phut, losing all my music (and loads of other stuff not backed up…).  Now all the music was still on the iPod, but unlike MP3s they are not designed to allow downloading back to the computer. All was not lost however when I found this:-

http://www.kennettnet.co.uk/musicrescue/mr_download.php

After loading the software (which is free, although donations are welcome), and CAREFULLY reading the instructions (essential not to have the ipod set to “synchronize with iTunes,” or you will wipe it!), I was able to get it all back.

So all is wonderful again!

…except that I re-loaded Windows XP using my daughter’s disk by accident, and it won’t register.

Now what do I do???

<<What do you mean by not being able to register? >>

I meant "activate"

“Windows must be activated in order to determine if the Windows product key installed on this computer is genuine.”

http://www.microsoft.com/genuine/downloads/nonGenuine.aspx?displaylang=en&cCode=GBR&Error=1&PartnerID=103&sGuid=c0bff2a0-6a03-4bc5-9bc6-1911046e9fb3

I cannot get the registration key that came with this computer (bloomin' Dell) to work with the disk I used - nor will it with the daughter's key (presumably because it is unique to hers). It won't let me re-load with the proper disk because it is an older version without Service Pack 2.  I think I will have to start all over again with a reformat - something that takes ages...

So I believe - for about 95 quid!  I think Microsoft have had enough of my money already, so I think it will be a back-up of everything and a re-format...