I guess it is possible that your friend could have installed something on the computer to spy on you, but I'd classify this as quite unlikely. It's also possible that they installed a piece of hardware on it for similar purposes, but this is indescribably less likely.
You could certainly hire someone to come out and do a system clean. If you don't have the original disks for any software installed on the system, you may need to re-purchase said labels. Windows runs around $100, Office suite runs around $200+. The labor is likely going to be around $100. And I promise that this request would not make their top 5 weird calls of the week. Trust me. I run just such a company, and this request is relatively normal.
So, in the end, the question comes to you: How paranoid are you? What might be your best bet is to pay the $100 labor, and get a professional opinion. I can almost guarantee that I could, in very short order, tell you if you have anything to worry about. Then, if additional measures are required, take those as necessary.