Line through circle on downloaded files

I just bought this mac and prior to picking it out it was wiped clean with 10.5.8 OS. Now although I can unzip a file, all of them have a circle with a line through them indicating that they are not useable. when I try to open them I get a message that says not compatable with this systems archetect or java error, although Ive downloaded the correct version for the os that I am using. Im using 10.5.8. on an intel 1.8 GHz PowerPC G5.
Any help would be apreciated