Everyone dreads the message popup on your screen. Virus Alert is usually accompanied by expletives and abused monitors. Well it is quite the Pain in the ass for sure.
First thing you should probably do is quarantine the suspected infectious computer. Unplug it from your network/disconnect from the wireless too! The following steps might just have you running again in no time. FREE products do a very decent job of cleaning infected computers however nothing is as good as a well qualified and trained technician. Get your hands on a USB Flash drive of atleast 2 -4 Gig. These things are dirt cheap and you can use the following files on multiple computers.
Download the following files and save them to your USB flash drive. Once you have them downloaded, install and run them against the infected machine. While it is disconnected form the internet.
Please note that AVG is the only actual Virus prevention tool. The rest of the linked tools are for removal of malware, they are the best. Good luck and if you QUIT LOOKING AT PORN, you might not need as much protection.