Hard Drive sapce is dramatically shrinking

Hi!
I ahve window XP, 55GB
Free space on C: drive is dramatically shrinking.

I started having this problem in January and followed the direction and thought it was resolved.
It started go bad again and I installed and used CCleaner.

But, now, eventhough I removed 200MB from program file, it disappears withing minutes.
Now, I cannot even eidt 1 page MS word file since it has only 0MB.

My antivirus program didn't find any threat.

Please help!

Thanks,