Hard Drive Memory Disappearing Rapidly

Something is eating my C Drive! I am running XP Pro on a Gateway 450 Laptop. Last week, my memory was down to 1%. I ran Zone Alarm's virus and spyware scans which resulted in the removal of an IE plugin. My free memory was instantly back to where it should be: 15 GB of 30 GB, which allowed me room to defrag my disk. Since then, it has been on a steady decline, but I have not installed or created anything that would result in such a memory reduction. Each time I check the properties of my C Drive, the free memory is substantially less. I am now down to about 1.5 GB. I have run Zone Alarm, CCleaner, NoAdware, 1 Click PC Fix, House Call and a couple of others that I have found mentioned on this forum. They have fixed the errors found, but the memory decline continues. This problem seemed to start when I replaced Norton Security (due to an apparent conflict with the Windows Firewall) with Zone Alarm. Any ideas of the cause? Please help!