I had to repair XP the other day and tried removing then reinstalling uxtheme. Now whenever I try to change to a custom theme it doesn't work. In the preview bit it just shows a thin blue line where the title bar should be. If I try installing the theme it just goes back to the Win98 style. Any ideas how I can get it back?

It sounds like the hacked uxtheme.dll didn't get fully replaced, try doing it again. The "original" one got put there instead when you did a repair, since it wasn't a verified system file.

After repairing, core system files are replaced with their originals from the CD. You need to reinstall your patched uxtheme.dll file.