oh yeah I forgot about ISPs doing basic virus protection
Of course most ISP's do email virus protection. They will not protect you from a virus being downloaded over other channels, such as the www, usenet, IM, IRC, and others.
And then there are the worms that use windows vulnerabilities to spread. You can keep those out to an extend by blocking a whole bunch of different ports, but doing so also prevent legitimate connections from getting established.
e.g. blocking port 1433 and 1434 to keep the SQL slammer worm out also means you can't connect to said SQL server remotely yourself.
blocking port 139 to prevent some of the netbios nastyness also means that you can't remotely map a harddrive yourself, etc.
Because of that (and many others), most ISP's will not block all of those ports by default, so you won't be protected against virusses that exploit them.