My question is quite straightforward. Can a virus or trojan etc; in any way affect hardware i.e. cooling fan or hard drive causing it to ultimately break down? All the computer experts no doubt will be laughing at my question, but a simple no would suffice. Thank you. Spadge.

No. Viruses only attack software, and cannot damage physical objects; but to the extent that hardware is dependent on software for proper functioning, viruses can effect their performance.
Regards,
John

I do think it is possible though for a virus to affect the software controlling the main CPU fan but I don't think it could affect anything BIOSish

I thought I heard that there were BIOS viruses...? Isn't that why Nortons has a boot disk scanner for when you shut down your PC..?

I think that is to check for Boot Sector Viruses.

A BIOS can become infected - but it's gotta be done using the same methods that involve flashing the BIOS - this doesn't prevent changing the settings in the BIOS with a software program. But, IMO it's way too much effort for too little gain - not something that most script kiddies would try doing.

Viruses can cause BSOD screens - and a BSOD is given when your system is shut down to prevent damage to the hardware - so a virus can cause damage this way also.

MBR/boot sector viruses are resident on the hard drive - not on the motherboard.