adult porn is legal as long as you or anyone who can veiw it on your computer is 18 years old. Child porn is highly illegal and sick as is anyone who views it.....IMHO
i doubt your isp will not be able to take action against anyone because as far as they are concerned YOU will have been on porn sites and have picked up the virus via a link/pop up/pop under/ vb script etc......
not sure what you mean by a web filter, a good anti-virus checker and firewall should protect you from virus's/ being hacked etc but the biggest threat to your computer is yourself....you have to take care about what you visit/download and also what you do with any e-mails you recieve.
if you dont have a virus checker, click here and download the FREE avg.
click here for a free firewall
other people will advise other products, its simply down to choice which one you use.
also try to use a pop up stopper if you do visit porn sites, it will be invaluable.
windows XP SP2 has a built in popup stopper but i use pop-up stopper pro.
good luck with future browsing mate.