I got a Sony laptop in Easter and it came with Windows Vista Home Premium. I'm not sure if your program could work with Vista, but I think you should give it a go. A lot of people says 'Vista is full of bugs, it sucks!' but there is a site, and a pop-up message from the bottom right corner will tell you when there's a solution to a bug, but it won't be so direct, it'll just say that there's a Windows Update. I think Windows Vista is very neat and all the functions are great, it's much more easier to move around with the 3D slide thing. Most programs now have updates so you can use it on Vista. My laptop came with Adobe Photoshop Elements and Adobe Premiere Elements, I was reallly excited about it, and once I turned on my computer for the first time, I found that only the programs' installers were installed, after installing, I opened it but it says that it is not compatible with Windows Vista! I pressed search site for updates, and there was an update waiting for me to download, after downloading it, the programs were fine. If you got Windows Vista Ultimate/ Home Premium, you'll also get the Windows Media Center, which is a really cool thing. However it's your choice.
Now to answer your other questions, once you change your PC to Vista, you are able to switch back to XP, unless you don't have your Windows XP installation disc and key code. (I'm quite sure about that, but don't quote me because I'm not 100% sure since I haven't tried to)
I hope that helped!