I say those forum people are either right with the PSU reaching end of life or the graphics card.
Although this might be somewhat related, too, since you said you did updates before:http://www.zdnet.com/why-all-the-errors ... 000020628/
Hope you get it sorted, Con. Fingers crossed.
By the way, I don't think it's a virus (modern viruses usually don't try to harm your hardware as such but try to steal data or make your computer part of a botnet that can then be used for sending spam mails, attacking other computers and the like).
I also do not think that HoMM is to blame.
On a side note: Heroes of Might and Magic V seems to run very well on Linux using wine.