How to access the BIOS in bootcamp

I'm running Vista in bootcamp on my Rev A iMac. For the most part, it runs well.

I use it for playing games, and I was having some trouble with the sound "skipping" in Half-Life 2. According to their support forum, it looks like I should try enabling HPET.

From http://support.steampowered.com/kb_article.php?ref=4911-EICN-7411:

Enable HPET
If the issue still cannot be resolved, you may be able to resolve the issue by going into your BIOS and enabling HPET (High Precision Event Timer).

Click to expand...

My question is... HOW do I access the BIOS in bootcamp... how do I enable HPET?