Does the hard drive capacity decrease or the space available on it?
If the capacity is decreasing you have a hardware issue. Is the drive that is decreasing the one you are running your OS off of? If so try running it as a slave so nothing boots. Some software like "Cryptainer" have the ability to partition off sections of the hard-drive and make these partitions invisible to users. Mcafee, Norton, AdAware, and such wouldn't neccesarily claim this software to be malicious.
Also, do you have backup software running on your computer? If so they often partition off sections of the hard-drive to keep it from getting corrupted. Backup software would continue taking more space if set up to never delete backup points. This would be a very logical explanation.
If the available space on the HD is decreasing and I am assuming you aren't installing more software or getting 5mb of cookies and temp files, then it could be viral/adaware. I reccomend running off of your other HD and see if the same problem happens. If not, you must have something running on your HD that could be creating logs/backups or collecting data. Some semi-AI software like Dragon Naturally Speaking will build logs when you speak so that it can learn your speaking style better. If you have anything like that it is a possibility. If you think you have something running that is doing this, follow these steps:
Open windows task manager (alt-ctrl-del).
Goto the processes tab.
Resize the window so you can see all running processes.
Watch the field on the left where it says mem-usage.
This is the amount of RAM the process is using.
Watch these amounts closely.
If any are gaining constantly without letting up then you may have found a problem.
If you do find something doing this DO NOT BE STUPID AND END IT!!!
Explorer often does this but it is valid.
DO post the process name here and I will let you know what it is.
I reccomend emailing me for further help at
corey@truceco.com.
I check my email religiously (lol) and you will get a much faster response.
Hope I could help you! If so donate participation points to me!!!! I want to beat all others!!!