It will just look different. Nothing else unless you check whether there are any new features in Windows 7 that in XP for the software or which one takes up more space. Technically everything is the same, is just the graphics is better in Windows 7.
Certain older software will perform differently under newer versions of Windows though, for obvious reasons, otherwise jingda is correct. It's also down to whether you have a 32-bit or 64-bit version of Windows.