You may want to do some serious spring cleaning of your hard drive. There are a lot of random updates that happen in the background, and sometimes earlier versions of programs/addons don't uninstall automatically. If you have been updating software recently, you might have some leftovers clogging up your hard drive. You might want to do to install/uninstall programs, and nuke anything you don't use anymore. For example, you might check Java -- it periodically updates on my Vista machine, and there is a cleanup tool on its website that helps you get rid of older versions. On other thing - if you have been installing new software recently, windows creates a new restore point every time, which can hog some disk space. Restore points generally last a month or so, depending on your settings, so it should not be a long term problem. The only other thing I can think of is that you have a bot or something on your system that is not spotted by your A/V software. You might try getting a antirootkit program to do a deep dive into your system and clean stuff out.