Make sure you don't have a Virus or some type of Spyware infecting your machine. AVG Offers a great Free Anti-Virus product and I highly recommend Spybot Search and Destoryfor any spyware/muckware issues.
If you haven't already done so, try re-installing your Windows Media Player. This can sometimes correct driver and codex issues along with possible file ownership problems.
All else fails, contact us and we will try to help work through the problems with you.