From: http://www.winnetmag.com/Articles/Index.cf...?ArticleID=4529

To restrict users from installing software on one computer, you need to change permissions for the HKEY_LOCAL_MACHINE\SOFTWARE Registry key.

Run regedt32, select the SOFTWARE key, select Permissions from the Security drop-down menu, and select the users you want to set permissions for. The SOFTWARE key's default settings include Full Control for CREATOR OWNER, SYSTEM, and the local Administrators group. These users can make changes freely; they can add and remove software, use applications that have entries in the SOFTWARE key, and modify permissions.

To prevent users in the Everyone group from installing new software without preventing them from using the software that the SOFTWARE key lists, you need to remove their Set Value and Create Subkey permissions. Change the Everyone group's permissions from Special Access to Read. Then, users in the group will have only Query Value, Enumerate Subkeys, Notify, and Read Control permissions.

Hope that helps

Another idea would be Deepfreeze... it doesn't prevent installation but it pisses them right off after they restart
http://www.deepfreezeusa.com/