I have windows vista in my computer and i also want to to have windows xp on the side so that i could work with a software which is not supported by windows vista. I tried to do it but it says higher versions don't allow to install lower versions program. I thought to install windows xp first and then windows vista but can anybody suggest me how to install windows xp sevice pack 2 from windows vista......

I believe there is a very simple solution to this problem. Download and Install Microsoft Virtual PC on Vista (Virtual PC 2007 is available free on the Microsoft Website). You can install Windows XP on Virtual PC and run both OS's simultaneously. For more information go to http://www.microsoft.com/windows/products/winfamily/virtualpc/default.mspx

yes, if you have enough memory to run both Vista and XP simultaneously then Virtual PC is the best bet. Vista sucks up a lot of memory and when you use XP as a guest, you are supposed to allocate RAM amount to XP, taking it away from Vista. My experience is, if you have RAM less than 2GB, it really is not worth to use Virtual PC.

And yes you are right, if you want to create a dual boot system, then you must install XP first.

Check this out too. http://www.vistahunt.com/how-to-run-windows-xp-under-windows-vista.html