WIN 2003 Small Business Server Primary drive fills up

HI,

Windows 2003 Small Business Server was setup by one guy in the company.
He alloted C drive with 12GB. Now its filling up very fast within hardly 1 Gb available. Twoce when he was working here he cleaned up and made speace but never educated me on this.
I removed temporary files but no idea where the files are piling up. I would really appreciate anyone helping me in this.

Thanks
– A