Delete all the favorites, the browsing history, files and cookies to start fresh, I'm not on my Win 7 PC right now but it's in the menu at the upper right. To totally get rid of any evidence there may be some registry entries and the index.dat file but getting rid of those is way more involved then we can do here. Don't sweat them if your activities aren't suspect, somebody with the knowledge to get those out would have the ability to recover much of even the deleted info.
If you're totally paranoid about it you'll need a full set of recovery disks and a new hard drive or a Bart PE disk with a Military grade disk over-writer program included, again that's beyond the scope of the forum. They really aren't watching you that closely ... yet ... and if they are the unique MAC address of that PC has already been registered