Pull the drive and slave it. The more you turn the infected machine on the worse it will get.
However, that being said, I seriously doubt it is a virus stopping you from booting up. The virus has to live in an environment. The OS is the environment. Not being able to get through BIOS to the OS is not a functionality of any virus that I have dealt with. I don’t even know how I would start to get that to work (unless the virus had a built in BIOS flash functionality that would trigger if a base dll in it’s structure was scanned... hmmm....)
Rootkits are nasty buggers than can actually get into your firmware and/or boot loader. They don’t need to thrive in the OS itself. In fact, there are some rootkits out there that can embed themselves in the hardware and survive even if you swap out a new hard drive and start from scratch.