iScreensaver 4 supports windows XP (see http://iscreensaver.com/requirements.shtml for the details ).
However, Windows XP itself is nearing the “end of life”. This means the Microsoft has no further plans to support it, and is strongly encouraging all customers to upgrade to Windows 7. Read about the policy here: http://www.microsoft.com/windows/lifecycle/servicepacks.mspx
Microsoft has stated that Windows XP SP3 is the last service pack update “No further updates planned”.
What does this mean for iScreensaver?
XP does not fully support hardware-accelerated windowing, and it never will. For this reason, performance of iScreensaver 4 under Windows XP will always be lower than on PCs running Windows Vista, Windows 7, and Mac OS X.
iScreensaver 4 is optimized for the next generation of computers and operating systems that support modern technologies. It will work under XP systems that meet the requirements, but due to these issues, it’s always appropriate to fully test any screensavers under XP to ensure that the limitations of XP are known.