I had a similar problem and my solution was to create a shortcut to appwiz.cpl on the Desktop. Not very elegant, but it got me out of a hole until I eventually got around to a repair reinstall of Windows (not a big job, but I was pushd for time).
You shouldn't need to do a repair install of XP, it might be just that your appwiz.cpl file is corrupt in some way, replacing that [possibly corrupt] file with a new version from your C:\WINDOWS\ServicePackFiles\i386 folder, or extracting the file from your XP CD, will fix the problem. The new file should go in the C:\WINDOWS\system32 folder. But you haven't explained exactly what has happened yet, answer Fruit Bat /\0/\'s question above. What happens when you double-click on the Add/Remove Programs icon in Control Panel? Doing a System Restore to a date prior to the problem occurring might also cure it.