Weird Internet Explorer problem

My sister's family computer (XP Home SP3) has IE7 as its browser. Suddenly Flash won't work, no matter what we do. I've tried uninstalling Flash and reinstalling it, tried messing around with the settings, nothing seems to help. Flash is there all right - I tried it with Opera and its's fine - but for some reason IE7 doesn't want to know.

Any suggestions? Preferably not involving Linux, Firefox, Macs, etc....?