Other users cannot run programs Win XP software signatures now unknown

Overnight, some odd things happened.
Other than my named administrator account, all other users of my Dell box (Win XP Pro SP2 full patched) lost the ability to run any .exe file ("you may not have permission..."). Upping them to administrators failed to solve.
IE 7 disappears if one tries to access House Clean etc. NAV 2007 fails to find anything, and Live Update cannot operate because it reports the "binaries may have been tampered with". Cannot reinstall, because the signature on the CD is now "unknown".

AdAware doesn't pick up anything, but it too cannot update. Trawling the web on the office computer doesn't reveal anything. Am now attempting to back up to DVD so that I can scan them with the office machine.

Any suggestions as to what has happened?