Hard drive fills just sitting there

I'm working on another old computer for a friend... Windows ME OS. It has two hard drives, one, the C: drive, has 14gB, the other, the D: drive, has 4gB. The C drive is constantly giving a low space warning. It has the OS and some of the programs on it--but not enough to fill it up. It seems to fill up as it runs, no matter how much I delete. I've probably deleted five or six gigs of stuff and it still says it's filled up. I switched virtual memory to the D drive, but that didn't help. The computer didn't have an anti-virus program on it and was used on the internet. I put Avast! on, and ran it, including archived files. Deleted three viruses. I tried TreeSize, but it won't work. Says that the C:\ and D:\ paths are not valid for the program (?!).
Anyway, very strange.
Also, it uses a dial-up internet with Juno. Every few minutes a dial-up screen (the Windows one, not the Juno one) will pop up. How do I turn that off?
Any help appreciated.
Keith