Viruses come in all shapes and sizes. One might give you annoying popup ads, while another might silently steal your data. Knowing you have a virus isn't always easy, which is why I've given you 5 signs to watch for.
Once you know you have a virus, removing it usually isn't too hard. You can see all the steps here.
Unfortunately, some viruses just don't want to give up. They might even lock you out of your computer completely! How can you possibly fix it?
If your computer won't even start up thanks to a nasty virus, you should try using AVG's Rescue CD. AVG anti-virus is one of my recommended free anti-virus utilities. The Rescue CD is AVG's prescription for really bad infections.
It's a set of tools that doesn't require your operating system to run. It can boot from a CD or USB flash drive and unpack itself to clean your computer of viruses without having to reformat your drive.