My alma mater, CodeJacked.com, explains how you can shave 30 seconds off of each reboot by holding the shift key while restarting Windows XP or Vista.
Sometimes, you need to reboot Windows (e.g. when installing new software), but there is no need to restart BIOS, too. However, the default is to reboot both. (That's called doing a "cold boot," rather than a "warm boot.") There's a trick that works on both XP and Vista to get it to do a warm boot instead, thus saving you 30 seconds per cycle.As a heads-up, if you've got a complex hardware setup you could possibly run into some trouble with a warm boot. If any BIOS experts out there can shed some light on when you would (and would not) want to use a warm boot, let's hear it in the comments.
Restarting Windows Without Restarting Your PC (Vista or XP) [CodeJacked.com]