upload but restrict

Greetings friends, pals, nymphos, programmers, geeks and others, of gods chosen people! Here goes, I have a program that uploads any file and allows the person to see whats in a particular directory without any problems, not bad for a newbie eh? stand up and clap!!! what i want to do is make sure that whatever the person uploads cannot "run" or be executed....any ideas on how to do this? Baically its a file sharing program..."a" uploads something "b" downloads it...I just dont want to get screwed in the bargain I was thinking of using the promise system, make them promise that they wont do anything bad...but the fact of the matter is people lie.....:-) Any help appreciated. Cheers, -Ryan. P.S after reading this email dont send me mails telling me that I am crazy.....coz i already know that. And remember "Never get into fistfights with ugly people, they have nothing to lose."