Well, if you're clever enough, you should be able to tell your parents your computer has a virus without telling them the specifics.
Computer viruses are common and can be picked up in many ways especially if you don't have anti-viral software.
So, I don't think it would be necessary to tell your parents you got the virus from a porn site.
Besides, it doesn't sound like you are certain yourself.
There are some free anti viral programs available. You may want to check CNET.com and do an internet search for them.
How good they are is questionable. But, it's worth a try.
If you can't find anything, the only thing you can do is completely wipe and reformat your hard drive, and of course, it wouldn't be safe to back up your data because your backed up data would probably be infected and you would only re-infect your computer when you restored your data.
So, you're better off talking to your parents, telling them your computer has somehow picked up a virus and you need to get anti viral software.