dferstat: No game should EVER require admin-level rights to run.
korell: That's just wishful thinking. I have some games for which the save system doesn't work unless the game executable is run as admin, and more than that, because they are on my Steam account I therefore have to run the Steam client as administrator else the game executable can't see Steam.
Unfortunately, for old games, if they need admin rights then they need admin rights. It's a minor incompatibility issue as they were designed for old Windows OSes that didn't have UAC and DEP. Modern games should take into account Vista and Win7 security and run fine, though.
EDIT: Oh, and for reference, my Steam client and games are OUTSIDE of Program Files (x86) and still there are games that must be run as admin.
No, it's not wishful thinking. Of the over a hundred games I've bought over the last decade, Thief 3 is the only one that has refused to run on an XP user-level account. Clearly, then, it's not impossible, or even particularly difficult, to do.
As far as Steam is concerned, I had no problems when I used to run Steam on a user-level account. Admittedly, this was about the time Half Life 2 appeared, so presumably we've had more time for lazy developers to release bad product.
"If they need Admin rights, they need Admin rights." The only old games I've had a problem with in this regard are, amazingly, those from GOG. Think Duke Nukem, Blood, etc. GOG's installer, for reasons unknown to me, requires admin-level rights to run (whereas the installers for Star Trek Online, Champions Online, EverQuest and others don't). A consequence of this is that all the config and save-game files are owned by the admin, and users can't edit them. The games themselves still run, though.
Games written for modern-multi-user OSs (into which class Thief 3, from 2004 falls), have no such excuse. They should not be accessing any files which require admin-level access, and they should be able to write to the appropriate users folders.
dferstat: Also, if it requires admin-level rights, how can I let my son (who, logically, does not have admin-level access to my PC) play it?
Waltorious: Can't you just set the game to run with admin rights even on a non-admin account? That won't solve your security concerns but it would let your son play it.
Unfortunately, the requirement for admin rights is more of a problem with Windows than with the games, I think, especially for older games. Games need to be specifically programmed with the new security features of Vista and Windows 7 in mind in order to run successfully without admin rights. Older games that were made before these OSes were released will often simply not function without admin rights, through no fault of the game developers.
As others mentioned, avoiding the Program Files directory when installing games will help minimize how often you need to run with admin rights.
I'm running XP Home, and there is no such facility to pre-set admin-level rights when launching an application. Thankfully, Thief 3 is the only game I've found that mis-behaves this way, though I read mutterings on GOG's Chaser forum that that game might share the problem. I haven't tried to run it yet.
Oh, and I don't install my games in the Program Files directory anyway. They have their own drive. :)
Unfortunately, there is simply no way I'm going to be able to upgrade to Windows 7 any time soon.