Thanks everyone for the advice. I stupidly forgot to list my system
specs - I have an nForce 2 motherboard so disabling the integrated
graphics wasn't an issue, it happens automatically when it detects an
AGP card.

I don't have any XP disks at all, but I have solved the problem by
borrowing a friend's copy of XP Home. I went through the install
process and opted to recover a previous installation. All appears to be
as it was before! I've now successfully installed my new 6800GT.

Two quick questions:

1. Windows is now prompting me to activate. AARRGH! I've used another
XP CD with a different Product Key. I suppose technically I've
infringed copyright, although all I did was recover a previous
legitimate installation. What should I do? If I call Microsoft are they
going to send round the boys? I can't remember whether I activated my
previous version or whether it was activated when I got it. The vendor
is Watford Electronics, which is well known in the UK.

2. Would you mind telling me if these benchmarks are about right?
Decent video card but mid-range system.

3DMark01: 15962
3DMark03: 10169

System is:

Athlon XP 2800+ Barton
1Gb Generic PC2700 RAM
Win XP Home
XFX 6800GT AGP
Tagan 480W PSU
Many thanks for all your kind help getting me going again!

C