If you've reformatted and it still happens.
You stuck on a bunch of antispyware etc and it still happens (as long as the antispyware isn't actually spyware itself I didn't recognise some of those names)
you've mailed from another computer and it still happens (might be wrong there I'm losing the plot a bit)

It is possible that the problem is at the other end? Does it happen when you email any address or just your friend?
On your friends PC. Are you sure that machine(s) is clean?

Check with the law before you do anything, everything you do has the potential to ruin any evidence.

Some general thoughts:

You work machine is possibly compromised. Have you spoken with your IT department?

Got private things to send/say to your friend? Write a letter/text/phone but don't use the PC.

Send **** everywhere by email, email the pope, billy gates anyone stupid. If nothing else it'll piss off your snooper and they might get bored for a while.