It's possible that you have some malicious program running and hidden on your system. Might also be that your hard drive is really bad or fragmented. A defragment wouldn't hurt. A scan for any malicious application is also good.
yep basically follow warez monster's guide (link is in his sig) the software he suggests should sort that out soudns to me like virus or spy/adware and also as some1 else has said a defrag might be of use id use O&O defrag