I found this on another site, might want to give it a try:
"Vista introduces a new concept to managing your audio settings. Instead of having a collection of fixed settings, such as Line In, Mix, etc., it dynamically updates the volume controls letting you control each application individually. This is a huge benefit, but may be the source of your problem.
To check this, open Control Panel->Hardware and Sound, and leave that window open. Next, launch Internet Explorer, Windows Media Player, or whatever application you are using to access the content, and begin playback. While it's supposedly outputting the sound, click "Adjust system volume" from under Sound in that control panel window...you should see a volume control for that application. Try adjusting the bar for that and other running applications, as well as clicking the speaker icons underneath to toggle the sound on and off.
If that doesn't resolve it I would consider missing audio codecs, but check out that one first.
"
BTW, I've found it to be a good practice to keep data and the OS on separate drives or if you only have one drive, split it into two partitions. That way you can always re-install the OS without loosing your data. For example, my laptop has a 120GB drive, with the C partition 20GB and the D partition the remaining space.
Another tip is to keep your Outlook PST on the other partition/drive as well. That way you won't loose contacts or email.