Hard drive appears almost full incorrectly

My Toshiba Satellite A205-S4577 has 150GB hard drive and Vista OS. I have put onto it few regular softwares, some photos, music and videos(totalling about 40GB), downloaded a few things – you know, nothing major. My hard drive has 15GB of free space! How is that possible? Oh, and another interesting thing – yesterday i had 25Gb free – then i cc cleaned it and defragmented it and now it shows 15Gb free.

HELP! short of formatting the drive (which i would really prefer to avoid)what can i do? I don’t believe the OS and software is taking up almost 100GB of my hard drive!