Virus?

My computer sometimes starts up, but most of the time goes to a blank screen, or a multi-colored screen. However, if i’m lucky to get it started up it doesn’t last long. The screen flashes like it’s turning off, then comes back on and is frozen. I can restart it and sometimes get back to the screen, but it just does it again. What can i do to get rid of this possible virus, or at least allow myself enough time to copy my files from the computer?