I am faced with a mind gobbling situation

A couple of days ago, the amount of sace in my primary partition was about 20 GB. And i just noticed that it is 11.2 GB now. Being somewhat of a power user (geek kid - and Harry Potter freak), it made me go "CODSWALLAP!" Anyway, i'm confused, i like to know what's using up the space, etc, you know. I removed the temp. internet files (as if they were gna free 8 GB ) and also removed all but the most recent restore points. The numbers sit at a proud 11.6 GB now.
I don't remember installing anything. My system is set to hibernate but that's only supposed to take about 300 MB ONCE. and it can't just randomly consume 8 GB in 2 days when it's following the same routine for ages.
So: Any ideas, theories, formulations, thoughts, suggestions, queries, something, ANYTHING?

And I'll keep on my hunt for massively big files!
Thanks!
btw, ignore the cheesiness, just finished annual exams