Well the option is there right with xp. I didnt even google for this
Goto Start -> Run -> gpedit.msc
Under the Local Computer Policy:
User Configuration -> Administrative Templates -> Network -> Network Connections:
In the right side, look for "Ability to Enable/Disable LAN Conection"
Right Click -> Properties -> "Enabe" -> OK
Btw, do play around with gpedit.msc when you have some free time ok, lots of nice stuff in there