No, I don't expect there will be a proper business style domain with domain admins when a few mates get together for a LAN party.
What I meant was you are using machine A, which is on the same network as machine B, do you know the local administrators password on machineB?
if you know that, and the location of the files you wish to delete then you can do what you want really easily.
Alternativly if you want to be a real bastard. check out http://dban.sourceforge.net
and securly (irrecoverably) erase all data on your enemys computer whilst they arn't looking.