|I have a XP Pro installation disk|
This is you set-up disk.
Reboot the CD again and ignore the first Repair option you get. Continue and wait until the set-up has identified the existing installation. You will get an option to install or repair. Select R for repair. This wild do a repair install, correcting the registry entries that are causing the problem.
This will keep all your existing programmes and data.. What you will have to do though is reinstall any updates and Service packs that did not come with the CD.