Sometimes, issues like this could be caused by a highly fragmented drive.
Or, possible something making heavy use of the HD in the background.
Since this is something that can be easily remedied, have you tried defragmenting the drive?
Sadly, if there were a few issues with Vista, and you simply done a in place upgrade to 7, chances are those problem were just carried over to the new installation.
The best way to upgrade, and be certain no other possible issuies are carried over, is with a clean istallation. Which means formatting the hardDrive and starting with a clean slate.
Just be sure to back up any personal DATA you do not want to loose first.