Flaky HD: XP Pro reload

[jash]

Hi
I've got a P4 based computer where the HD seems to be flaky. I've got dual boot 2000 & XP. If I try XP, the computer wants to scan the disk, reports its ok and then repeats the same process, never going on to load xp.
If I try 2000, the system asks for a disk scan but then will go on to load 2000 (on Partition 1) and I can read what is on partition 2 (XP).
I'm thinking that I need to put in a new HD, keeping th old one for reference & reformatting at some point.
Question: If I install XP on a new disk, what do I have to do re activation and will it fail for that reason?

I'm also going to swap the IDE cables between the DVD and HD, as I just wonder if the cable is dodgy.

Any other suggestions?