Using VISTA on laptop, xp on desk and

wondering if I can put the laptop Vista OS onto my desktop because I do "own" Vista on one computer. I am not technical and some of the tutorials are over my head. Sorry, just the facts.
I want to keep XP on desk, but Microsoft is making that not worth it. But, since I have Vista on my laptop, can I use THAT OS installation and put it in my desk top?
WOW, I know that's going to bring a lot more issues, ques, etc. but my only option was to by Win 7 and probably have a company/tech do the work. I've never even backed up data, files, etc. Since I'm going to make a large change, I'd like to take the simplest path and would greatly appreciate your help. Please remember, I am not techie, but I'm always willing to try and learn.

Please advise. Got to move fast so I don't have problems with still having xp on my desk top. Thanks ahead of time to those who can advise.

Read the article here: Windows XP Isn't Safe to Use Anymore. Here's What to Do Next.

Good information, well presented. No hype or scare tactics, just useful info and good ideas on what to do.