Do you still have the option to uninstall QuickTime via Control Panel > Add/Remove Programs? If so, try this:
1. Open Control Panel.
2. Select Add/Remove Programs.
3. When the window with the list of programs available to uninstall comes up, before
selecting QuickTime to uninstall, close Control Panel by clicking on the X in the upper right corner of its window.
4. Then select QuickTime from the list of programs.
5. Click on "Uninstall everything" (or however that option is worded in XP).
6. If you're asked to reboot, do so.
7. Run Windows Registry Cleanup Tool (RegClean.exe) as many times as necessary until it says it found no errors.
If XP doesn't install RegClean.exe, it may still be available on Microsoft's download site.