I never had this problem before putting the HDD's on SATA mode in BIOS (actually, I had them put on AHCI), and didn't really need to enter BIOS after that. But now I need to, and the damn POST goes too fast! I know that for Gigabyte S3 motherboards the BIOS key is DEL, so I keep pressing it, press really fast, or any variation of those two actions, but nothing happens. What do?
Couple things you can try:
1) Turn off the system, put your hand flat on the keyboard, effectively holding down a bunch of keys, and turn it on. You will hopefully get a 'key stuck' POST warning which may help get you into the BIOS.
2) Take out the CMOS battery for a while to reset the BIOS to defaults. This will often cause the system to stop at the BIOS on next boot so you can set things like the date and such.