Yep, start with UBCD DT, check mem and HDD for errors. If you get no errors on the basic scans, run the intensive ones. If you get errors, get the offending components swapped out. You don't mention if your machine is under warranty, but even if not replacing the HDD and/or memory is not all that costly. If you don't get errors, back your important files up and reinstall windows. Make sure you have av installed before you go back on the web. DL the necessary drivers beforehand and stick them on your external HDD.
The BIOS hang is worrying as that means the machine is not passing the POST (power on self-test). Try the things above, and if no better, flash the BIOS, and if issues still persist, I'd be looking at mainboard/CPU faults.
Jabo