Just a friendly word of warning before you replace Windows XP with Windows 7, Lynne326.
I did the same thing recently on my 11-year-old Dell desktop computer. I ran the Windows 7 Update Advisor (as 123Runner very correctly recommended that you also do) and luckily everything was fine there - so I went ahead and I am really extremely pleased with the results. I would certainly recommend that you do it, if you can.
My one proviso is to warn you about possible problems with the Windows 7 Key which you will need to buy in order to have your copy of Windows 7 authenticated. I bought mine from what I believed was a reputable website, indeed it was a site that had been mentioned by somebody on this forum as being okay. However, when I came to try to use the key, I was told that it had been blocked as it had been used on more than one previous occasion. I ended up having to contact Microsoft Support and one of their online helpers actually took control of my computer and I had to find and show them the email which was proof of legitimate purchase of the key. Luckily, all was in order and they then gave me a new key. I had the exact same problem last year when I bought a Windows 10 key for my daughter's old laptop which I had taken over: I used a different website for purchasing the key then, so it seems as if this may be a very common problem.
I have absolutely no idea how you can be certain that you are purchasing a key that will not incur the same problem that I had, but whatever you do make sure that you retain the proof of purchase.
Good luck with this. I would be really interested to know how you get on, so please keep us informed.