I don't have the problem, but then I always turn off the setting to remove unnecessary files. You may want to see if running Disk Cleanup from the Start Menu in Windows and selecting the options for types of files to be cleaned up can help eliminate the problem. The options in a manual run of Disk Cleanup (Start/Accessories/System Tools/Disk Cleanup) are (or were the last time I checked) respected by the OneCare Tuneup execution of this.
I just ran it manually on this Vista box (2 weeks old, never ran cleanup before or looked at the options) and see that thumbnails are checked by default. Uncheck the thumbnails option, run the cleanup, and then see if this is respected by OneCare Tune-up. I believe it will be.
-steve