linksys firewall
I'm assuming you mean a router with a firewall. Wired or wireless.
If wireless are you running any security on it?
If so are you changing the keys regularly.

You formatted the drive which should have removed any nasties but he could still see what you were doing. This should point to the problem not being on the PC.
I.E.

a) someone could monitor a wireless link if you have it
b) someone could be looking at your traffic on the ISP side.

Change ISP and hardwire yout computer to your net connection.

AND.... got to agree with wiskic10_4.

Why would a stalker monitor you and email your friend?

Are the authorities taking this seriously? Have they offered to image your drive foresically to preserve evidence? Have they contacted your ISP? Have you contacted your ISP?