Vista required a high-performance machine with full graphics acceleration to run on and that's exactly why most of the applications made for Windows XP do not run natively on Vista. However! a lot is going to change with the release of Windows 7.
Even though Windows 7 won't still be able to natively run Windows XP apps, it will however be able to run them in a coherent mode using virtualization.
According to a new announcement, Windows 7 will feature an 'XP mode' that provides one with the flexibility to run many older productivity applications on a Windows 7 based PC. Means you no longer need to worry about software compatibility while deciding to switch from XP to Windows 7. Here goes a preview of the Pet Lookup app for XP running in Windows 7.
All one needs to do is install a particular application in XP mode while within Windows 7 and the application will be pushed to the Windows 7 desktop. It will make you run older apps just as you would normally.
A beta of Windows XP Mode and Windows Virtual PC for Windows 7 buils will be made available soon by Microsoft.