Can no longer send e-mail in Outlook 2003?

I can receive e-mails in Outlook 2003, but cannot send.
Error message reports that my Port 25 is blocked (error message: 0x8004109).
Checked the same account on an XP machine using same POP3 and SMTP servers and router and no problem - can send and receive OK.
Did the same on another VISTA machine, with same settings and works OK.
Added a different account to the non-sending VISPA PC in question and it reports the same error.
Problem is definately isolated to my desktop PC.

The problem started about a month ago, and prior to that, all systems worked perfectly. Unisntalled AVG10 completely. Disabled Firewall. Uninstalled all updates for last 2-months. Created inbound & outbound rule to open PORT 25 (using mmc.exe in CMD mode) - no difference

Port 25 does not show using the "netstat -an" command.

ISP reports that all systems are working correctly on their side!

Any suggestions before I take a hammer to the PC?

We had a similar problem with McAfee which blocked port 25, so could be anti virus.