It isn't necessarily due to
w32.blaster.worm - my firewall regularly gets scanned by all sorts of things from old-fashioned ping to the latest worm-of-the-week. It almost certainly isn't an attack as such, just an attempt to find out what is out there in your corner of the internet. If you're exposing an exploitable vulnerability then you may get revisited in more depth, but your computer is probably too dull to be of interest.
Your firewall is doing it's job, so unless you are repeatedly scanned across a variety of TCP ports then i'd ignore it. A precursor to a real attack would probably be sufficiently stealthed that many firewalls wouldn't pick it up anyway.
As an aside, if you're really interested in learning about what's going on, have a look
here, have a look at some of the reading list, grab nmap (easier if you have a Linux box) and start scanning yourself. It's quite interesting what you may be offering to the outside world