html problems

Apparently my site is schizophrenic, and the css which has been working fine for over a week now, seems to have decided (if you're on Win. IE) that really, it doesn't want to work anymore. Considering I haven't touched the souce code for a couple days, and the problem only started today, I don't know what's up with it. Once my Eng. oral is done, I'll try and fix it.