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First of all, as it's stated in our rules Web Site Terms and Conditions of Use and EULAs of games sold here - our customer can only install and use them legally on his own machines. It does not matter here how many copies of the said software the user owns.
On the side note EULA is a form of a contract. :)
If you want to dig in the legal stuff than please note that you don't own any software and are only granted the right to use it (yes, that's what you really pay for).
There has also been posted a question concerning buying multiple games on one GOG.com account (aka paying for the right to use it... er... multiple times). When you make a purchase on GOG.com you can have fun with a game on all of your machines. So there's no real need for you to pay us multiple times... unless your not an end-user. Due to our deals with the publishers we can only provide our service to end-users.
When you install our game you have to agree to our EULA so it's seen as something (you might have noticed that I'm not a lawyer) similar to signing a contract with GOG.com. And contracts are not to be violated. :) In any case according to the EULA you are our customer. Your friends that would like to play the game on a computer that is not yours are not our customers and thus cannot play the game legally (unless they'll create an account and purchase the game they want to play).
Anyway... What we have here is a nice and user friendly place of internet distribution. As we also have be publisher friendly so that they'll agree to our DRM-free policy this state of affairs is not easy to maintain. Just consider this the next time you'd like to ask a question concerning breaking our Terms of Service.
edit:
Unfortunately I forgot to answer OP's question. You can install a game on any of the computers you own. If your friends are going to bring their own machines to your LAN-party (and this is most likely) they'll have to create their own accounts on GOG.com and buy the right to use our games.
Post edited June 29, 2009 by Tirpitz
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Tirpitz: If your friends are going to bring their own machines to your LAN-party (and this is most likely) they'll have to create their own accounts on GOG.com and buy the right to use our games.
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MGN001: So, if my friends use my computers, we can, for instance, beat co-op Duke Nukem 3D: Atomic Edition at my LAN event?
NOTE: wouldn't need the DN:AE since I already own a copy on a floppy.

You're asking about a game purchased from GOG or somewhere else?
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konoharaven: What if my friends gave me their computers for the night of the LAN party, and I returned them at the end of the night with my software taken off?
Then, technically I own all the computers I have any game installed on, right?
Yeah, that is pretty sketchy, at best.

Hmm... It seems that you'd make profit (getting their machines) because of actually letting them play your game. And in that case you wouldn't be a "end user" thus making the whole thing illegal as we can only provide our service to end users. ;)
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Tirpitz: You're asking about a game purchased from GOG or somewhere else?
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MGN001: Purchased on GOG.

If their really your machines than it's Ok.