...I'm assuming you're using Active Directory as opposed to a stand-alone server. Launch Active Directory Users and Computers and right click on the domain. Select 'Properties' and click the 'Group Policy' tab. You should see a 'Default Domain Policy' there. Highlite it and click 'Edit'. This will launch the Group Policy editor, and you can go to town. Of course, any policy you set here will affect everyone on the domain.
To affect only a certain group of users or computers, create an OU and drop users or computers in there. Configure OU policies the same way, beginning by right-clicking on the OU object. You should create a seperate policy for each OU.
Hope this helps.