Jeebus theres a lot of scaremongering going on here....
Has no-one thought its just the BIOS start-up screen that is being displayed and not the actual BIOS....
Right. 'dangle berries'. In a nutshell.
The OP has said he was downloading dodgy software from dubious sources. These downloads are riddled with malware, always are, always will be.
Whilst doing this, the BIOS configuration changed.
Coincidence? Really?
Its far more likely that the BIOS and/or the boot block (if its new enough to have one) have been compromised. The big danger with pre-boot compromises is the OS is unable to detect the compromise in any way, so immediately the OS is pwned by somebody nasty.
A BIOS update is probably unlikely to resolve it, due to a combination of poor software handling by the flasher, and because at the point of flashing, the system is still compromised. If the bootblock has been done as well, no chance.
Can it be fixed? Yup, most probably. I can certainly put the OP in touch with companies that can analyse and usually remove the infection, as I'm sure Guffer could too. An initial investigation is likely to be £20-30k (exc vat), with a quote then generated. For most people, a £150 brand spankers PC is a far more viable option.
Scaremongering? Nah, sadly its reality. Apart from a desire not to see a fellow OOFer get oppsed over by scumbags, I really couldn't give a shit about anyones PC. One more compromised PC on the Internet is irrelevant to me, as long as none of mine are.
To the OP, you've had the views of somebody with a 20yr professional background in IT systems engineering, and the views of our resident professional IT Security guy, and both (for once

) are broadly in line with each other - it is very likely that that system board, that hard disk and potentially that RAM will be controlled by somebody out to make financial gain.
I would strongly recommend to never use it again, and from another PC, change all your passwords used in any online services.