Help me Restore my pc?

I have a hand-me-down HP Pavilion 9795c that I’ve used minimally over the past few years and it worked just fine. However, the other day I finally got it connected to the Internet and the things crapped out on me. I had the IT guy I work with take a look at it and he recommends I do a complete restore. He is just too busy to get to it for about a week, I was lucky just to get him to take a look at it.
Can someone please let know how to do the restore?
Thanks