Not a legal eagle Tom, but it is well known that there are spammers with millions of e-mails. I have had my e-mail address used as a spam address, which pisses me off. There are many viruses that an infected computer will "grab" e-mail addresses off of a person's outlook address book and then send out, as if you had sent it. You can read on this on
www.symantec.com and other anti-virus (AV) sites, where they explain that stolen e-mails are used by nefarious people.
The point is, that it would be very hard to prove that YOU actually sent him an e-mail that "corrupted his system"...
I would either ignore it, or send him an e-mail and say, I have NOT sent you any e-mails (ever, the last two months, two years, etc.) and that there is no way I CORRUPTED YOUR SYSTEM.
More than likely they either have a "corrupted" system or they have old anti-virus and have a virus on their system, then you could say, hey, I have and always have had the latest AV on my system, so it can't be from me.
Ed