As a rule of humb, when reinstalling Windows you should update and install all anti-virus and firewalls you´ll need, BEFORE you connect to the net. Leave the cable unplugged, just in case. Then when everything is running nice and secure, then you can go online.
As for your particular problem, i had a similar one back in the day when i still used 56k. I had to wite to my ISP warning them that my connection had been tampered with. I didn´t want to receive a 1000 Euro phone bill. I discovered that the guy had changed username and was messing with my network.
Get a solid firewall and deny access to everything. Then as programs need access, you manually allow or deny permission.
If i were you, i would format again and start over. If you need something that you can only get online, get it first and save it somewhere, so that you don´t need to go online before your system is secure.
A good firewall and anti-virus is Trend Micro PC-cillin. Easy to use, always up to date, and goes easy on the system resources, unlike Norton.