I used to get strobed like this - warnings from my firewall every two or three minutes - until I installed SpyBot recently. After using SpyBot to clean a load of unsuspected and previously undetected spyware and then immunize my computer, I only rarely get strobed - just three times in two hours tonight. Even then its usually just the regular Slammer Worm stuff that's still floating around the internet, from PCs that have no virus protection at all.
I've a feeling that a lot of the intrusion attempts are the result of undetected spyware 'calling home'.