Unable To Set Compatibility

I'm currently running WinXP Pro (Build 2600) -- upgraded from Win 2000, and am unable to select different compatibility modes for programmes. It used to be that I could set it by right-clicking on the .exe of the programme I wanted to run under a different Win OS (Win 9x, NT etc.) and just click on the 'Compatibility' tab. That tab is no longer there.I also tried running the Compatibility Wizard (Start>All Programs>Accessories), but when that loads the buttons that should say 'Back', 'Next' and 'Exit' have no text on them. If I click the button that should say 'Exit' then the Wizard closes. However, clicking on either of the other two buttons does nothing so I am unable to go through the wizard.It's been several months since this happened, but I never really thought about it until recently because it wasn't until now that I wanted to put an old game on my system; but make no mistake, it's not just the game that won't do it, I can't set the compatibility for any program I have installed, or would want to install. System Restore doesn't go back far enough to try that, so I'm not sure what else to do. Possibly unrelated: At one point, I somehow managed to disable Task Manager; even manually replacing the file for it in the Windows directory didn't bring it back, although it did eventually return seemingly of its own accord. That may have been around the same time that I lost the compatibility ability, but I can't say for certain.