It might have been System Restore at work, does it still happen now? If yes, give this app a try to see what is taking up all that space:
JAM Software - Windows Freeware
And try an AVGas scan, see link and instructions here:
http://www.pchelpforum.com/118557-post2.html
Also try an online AV scan:
Run an online scan at Panda's ActiveScan - Please go here using Internet Explorer.
- Once you are on the Panda site click the Scan your PC button.
- A new window will open, click the big Check Now button.
- Enter your Country.
- Enter your State/Province.
- Enter your e-mail address and click send.
- Select either Home User or Company.
- Click the big Scan Now button.
- If it wants to install an ActiveX component allow it.
- It will start downloading the files it requires for the scan.
- When the download is complete, click on My Computer to start the scan.
- When the scan completes, if anything malicious is detected, click the See Report button, then Save Report and save it to a convenient location.
NOTE: Please ignore any entry(s) it finds, and the offer to buy the program to remove the entry(s), as we will address this later.
Post that log, and the AVGas log please.