The lazy man in me says 'viruses are a windows problem, let them deal with it'
The wishful thinker in me says 'we don't get no viruses'
At any rate, I believe sysadmins work on the principle that if you keep the viruses out of the mail, it is very unlikely that any others will find your system.
Also, if you are administering the domain effectively, then all the important stuff tends to be held server side, and hence will not suffer from the viruses. Because nothing important is held on the terminals, if something goes wrong you can just wipe it off and start again.
R