Showing Very Low Disk Space on C Drive

At first I am giving you a snapshot about my pc:
1. I have got Samsung 120 GB hard disk on my pc and more then 12 GB has been allocated to drive C.
2. I am running Windows XP Service pack 3.
3. I have installed only few software and as a whole it take around 5 GB place in C drive.
4. I am using my pc as a print server and more then ten users are sharing my printer.
After three months of clean service, from last week I am seeing a message that disk space is very low on drive C every time I start the pc. But I have never installed any kind of software on drive C. I have manually checked the drive but unable to find any extra files.
Do you have any solution of this strange looking problem?
