CS5 is not compatible with XP. XP was only available in 2 64 bit versions. Chances are you do not have one of these versions unless you specifically installed either Windows XP 64 or Windows XP Professional 64. Both of these versions only worked with certain processors.
On the Adobe site the System Requirements specifically do not mention XP so I don't believe that any flavor of XP will allow installation.
Rick is right. After Effects CS5 will not install on Windows XP.
As the system requirements page that Rick points to says, these are the Windows operating systems with which After Effects CS5 is compatible:
"64-bit operating system required: Microsoft® Windows Vista® Home Premium, Business, Ultimate, or Enterprise with Service Pack 1 or Windows® 7"
thank you what about cs4 will it work?
> thank you what about cs4 will it work?
After Effects CS4 does run and is supported on 32-bit Windows XP.