In this situation, a Mac would seem ideal.
No viruses or spyware even exists.
Video conferencing is literally 'one click' in iChat.
OS X is *very* easy to use for the inexperienced user (but is Unix so is ultra stable)
You have Garageband music software for free, which is amazing. More and more bands are recording in it. (Or something like Logic Express for less than £100, which is literally Pro quality).
Support wise, you have an 'Applestore' in most towns. If there is an issue, your mother could just nip to an Applestore, and they can fix and advise on the spot!
A Macbook these days is around £699 for the 2.1ghz C2D, 120 Gig HDD, 1gig RAM model.
The other good news is that it can also run Vista and XP natively, so you have the best of both worlds!
I bought my Mum a MacBook Pro a few years ago, and she has never looked back even though she never used a computer before then. Email, video conferencing to see her grandchildren in Holland, photo stuff, you name it... and hardly any support needed from me!
Imho, they get a bad press from geeks who like to tinker, but for the situation you describe, a Mac would be great.