CS5 will only install on 64 bit OSs. When you purchase CS5 it includes a copy of CS4 for users that don't yet have a 64 bit OS. Unfortunately, Adobe no longer provide a downloadable trial of CS4.
Your only choices are to (a) upgrade your OS, or (b) hunt around for a trial version of CS4 from another source.
well i have CS4 i just wanted to upgrade!! i am not good at it at all
As Andrew says, After Effects CS5 and later are 64-bit applications.