Software Installation

[peswar]

I have compaq presario r4000 amd athlon machine 512 mb 160gb xp sp2. After installation of sp2 from cd given by microsoft any other software downloaded fails to install wih the error unknown publisher and file corrupted messages. This happens even for genuine software like adobe, skype, ie8 etc But the samesoftware if installed through a cd goes through fine. I am not sure how to resolve this problem

[Tripredacus]

Specific error messages would help. Also see if anything useful in Event Viewer.