An update of Norton Firewall completely screwed up my WinXP recently. I had to forcibly uninstall Firewall and Antivirus using tools from the Symantec website.
I'm now running the free AVG antivirus (it works OK) and XP's own firewall (which is basic, but I've also manually disabled everything except my TCP/IP protocol and closed off a load of ports).
The Shields Up!! site (
https://grc.com/x/ne.dll?bh0bkyd2) reports all my ports as "stealth", so I figure I'm fairly secure...