mchain - ran memtest as suggested, but no errors as I expected.
I think I'm going to give up with this system and remove Avast, it obviously doesn't like something and I'll assume the support team have no ideas or they would have made a suggestion. I'm kinda disappointed that they couldn't even reply after requesting information, but I guess that running their software on such a dated system isn't very important to them.
Thanks for everyone's advice and interest, I hope you have more luck solving this error than I did.