Ideally I would install the new disk, make a ~40gb partition and reinstall Windows from scratch. Once up and running and you have reinstalled your apps I would stick in the old disk as a secondary drive, create a partition from the remaining free disk space on the new disk and then copy all your data onto it.
Once happy you can remove and discard the old drive.
I would set aside an entire weekend to do this as it rarely goes completely hitch free. If you do decide to get a new disk and you cannot find your old Windows XP disk let me know by PM and we can probably sort something out.