The problem is not AS 2.1 and XP. AS 2.1 works verry well under XP - without any problem - and on wide screen too. The only problem is, if you install AS 5 or higher may be for test, you have destroyed your AS 2.1 for ever - or you install XP on a clear PC. The higher AS versions overwrite something in Windows XP. From this moment AS 2.1 crashes for example while rotating or stretching.
It is also true that problem causes only with higher resolutions - so if you can work with lower resolution or with flat circles on a wide screen, you can get back your work.
It would be nice if someone would find the DLL or what else, which has been changed during the installation of a higher version of AS.