I have avoided becoming too familiar with Vista because I loathe it; but in your control panel there should be a listing for “sounds and audio devices.” Double-click it; on the “audio” tab, make sure that the soundcard you have is listed as the default device under “sound playback.” On the “hardware” tab, again check to make sure the proper devices are listed in each instance, & make sure none are listed as disabled, or having a problem.
When you have (virtually) uninstalled your sound device from the device manager (or whatever vista has pointlessly renamed it), and rebooted, what happens? Does your OS autodetect the device?
I have had random things effectively disable, override or remove my sound device to the point where I have had to reinstall from scratch many times. Now, I just keep the motherboard sound drivers (different from the separate sound card’s drivers, if you have one) in a zipfile because it can be a real pain trying to get them from the motherboard manufacturer. It might not be a bad idea to download the latest release of the drivers from the OEM, just in case your local copy is wonky.