You really shouldn't be alarmed (for the moment), the only time i got angry at portscans was when i noticed an IP that pscanned me at least 8 time a day (for 1month and 1/2)...

Download a proggie to see what is running on your box and try to see if u find any thing unusual. You can find a good .exe here.
Try to figure out if the portscan isn't being done by a website u often visit, by ur irc server (if u use it) - some of them scan the users for miscunfigured proxys and etc...

If you really want to do something about it, email the abuse handler at the ISP were u tracked the attack with the logs of ur firewall and see if they can at least flame the user (he isn't doing anything illegal but it is anoying ) and see if it's him doing the scans or if he is indeed being used has a zombie to fin more exploitable machines.