Is it possible to tell 2 different computers apart?

[cunoodle2]

Other than using IP address is there a way with php (or another web language) to tell the difference between 2 computers?

 

Here is a little more info:

I have a referral system in place on a site and I don't want a user to be able to refer themselves to get points.  I don't want to limit this to IP address as very often co-workers refer each other and they almost always have the same IP at work.

 

I don't want to use cookies as they can just delete them (cookies) and refer a new "fake account."

 

Any ideas?

[sKunKbad]

No, there isn't a way to identify a specific computer other than IP address, cookies, browser detection, OS detection, or a combination of all of these, and none can be trusted.

[cunoodle2]

No, there isn't a way to identify a specific computer other than IP address, cookies, browser detection, OS detection, or a combination of all of these, and none can be trusted.

Do you have any suggestions in my above scenario on how you would prevent users from referring themselves?  Any help would be greatly appreciated.

[freeloader]

No method is waterproof, but nothing is impossible either. You could run a java applet which would send you back their mac address and select based on that. It's not worth it I believe however.

 

Could store them based on flash cookies; harder to delete for most people than normal cookies.