[Skunkworks] XP, Vista force Proxy Settings

Morning Skunks, For the Windows geeks out there, Setup: I have a LAN with XP & Vista workstations. Problem: I need to force all network traffic via a proxy (for reasons beyond my control, we can't place the proxy "directly" between the LAN and router, so the proxy is part of the LAN). Setting the same in the browser settings is not "fool-proof" [?] The only possibility is to force all LAN stations to pass through the proxy...somehow... Is there a fool-proof way to set the default proxy/route in Windows? Also, how do I restrict LIMITED users from changing the LAN settings? All your help will be appreciated. G.Patronas.