Your system doesn't meet the minimum requirement of a 2GHz processor for playing videos from the iTunes Store, so there may be nothing you can do. But here are a couple of possibilities:
HP DV computers for some reason reportedly do not work well with the copy of QuickTime that is downloaded and installed with iTunes. Try removing QuickTime (not iTunes) with the Add/Remove Programs control panel. Then downloaded QuickTime ONLY, from the Apple website, and reinstall QuickTime.
There is also some discussion and possible solutions in this thread:
http://discussions.apple.com/thread.jspa?messageID=4555850�
Even if you do all of that, you may well still have problems since your processor just may not be fast enough to decode the movies smoothly. About all you can do is to make sure that no other applications are running and that you have sufficient RAM in your system (with Vista, at least 2GB) to optimize performance of your system and hope for the best.
Regards.