Mungkey,

Interestingly enough, my ISP does exactly the same thing to me every time I log on. I don't know exactly how it works but, the ISP knows I'm on, however I have a packet dropping firewall and there is no indication that a live node is on the other end. It is part of their software to acknowledge an IP is being utilized.

So in your case, I would contact them first before I decided to take some action. Also remember that although you may be getting scanned, it may not be alright for you to scan back.

cheers.

edit: Additionally, most of us get scanned frequently when we are online. Sounds like your firewall is doing is job of blocking or dropping. There are a myrid of scanning tools out their also. www.google.com

Here's a quote from Slarty, one of our Senior Members, and it's appropriate here as well:

Nearly all intrusion attempts since about 2001 have been from Windows worms. The owners of the machines are not (directly) responsible for their actions; most are entirely unaware of it. You should configure your IDS to ignore common worms intrusions, because they are so common and it is so pointless to report them.