Why am I losing disk space on my Local Disk C Drive?

HELP PLEASE!!! I am losing madd disk space on my dell (this computer im using now) I have no idea what to do I have like 20 MB left!!!! Please leave me a step by step process on what to do please! THANX
Update: im losing all this disk space for no apparent reason i havent downloaded anything ands its still losing space.
Update 2: Guys I tried my antivirus (kaspersky) it doesnt work I've tried add/removing things its just.....When ever I get space I lose it within a day its NUTS
Update 3: CCleaner helps but I lose it like 10 minutes after. System restore doesnt work cause I tried it and i kept losing space. Antivirus did'nt work. HELP!!!
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