My computer has a virus, and I need help?

My computer will not execute an executable file, and it's pissing me off. THe virus got onto my myspace account and left comments on all of my friends statuses etc. I'm trying to get rid of the virus, but I cannot download malwarebytesm, because as I said before, I cannot execute the file. CA Anti Virus will not run either. What do I do? Step by step instructions would be appreciated. x