I've been noticing constant HD usage for a few days now; also, the system cache increases in size until it takes up all available RAM and makes Windows unusable. I used a tool called CachemanXP to clear the cache, and it quickly filled up again.
I use AVG, don't use IE and don't download questionable programs so I don't think I have a malware problem. I didn't notice any strange processes/services.
Using Process Monitor I discovered that the system is constantly accessing various system files, the page file, and the MFT, I'm not sure if that is normal or not.