Tony (if it was you who asked the original question - sorry, my newsreader
only picked up this post in the thread, so I might be answering the wrong
question, or addressing the wrong person...)
Changing the processor will not normally trigger a reinstall or even a
reactivation of XP. If all the other hardware (motherboard chipset, etc)
stays the same then XP will not care - it'll just boot up at the higher
speed.
As the 2.6/800 Pentium 4 processor supports Intel's Hyper-Threading
Technology, ensure that you have Service Pack 1 for XP installed - this is
needed to get the most from HT. And with SP1, the presence of HT on the new
processor will be automatically detected; Windows XP pre-SP1 required a
kernel change or re-install in order to shift from Uniprocessor (non-HT) to
Multiprocessor (HT) mode.
Good luck!
-David Hollway
(mail as in header - remove spam blocker)