How to make our computer free from viruses?

I have experienced difficulties in managing my PC now. I found out that some of my important files were lost, eventually crashed, and performed slow.
There's something happening in my computer, that I don't know. Since, I am the only one using it and I can't remember if I manually deleted my important documents. Is there a possibility that a virus is the cause?
If this could be the cause, how to avoid the virus?
I mean, how to make my computer free from viruses? Is there anybody who can give me idea on how to fix my computer?
Please help me.
