Computer Is Often At 100% Disk Usage

[Tribal Chiefton]

My PC has recently started to run slower and the disk usage has commonly been at 100% usage for a bit now. I've tried various quick fixes like stopping windows search, superfetch, and changing my virtual memory. I'm not sure what to do now and it's quite frustrating. I also doubt it's due to a specific program using up my dis since sometimes it can have nothing running and be 100% whereas other times I'm running a lot and it's a low number. Any ideas on what it is? Is it a hardware problem?

Computer is about 2-3 years old

 

[Cycloid Torus]

Candidates:
- too much stuff on HDD - use cleanmgr
- too many chopped up files - use defrag
- two anti-virus programs - clean out both and install and use just one
- start taskmgr and monitor disk use by process (quit torrents and other 'internet sharing')
- scan for malware (trojan &/or rootkit)