Your game's objective is to keep all the present computers clean from Wingdows [TM]. This virus is spreading and you have to stop all its eval servants by left mouse button (clicking to the empty place is useless but it doesn't cut up you in any way). The game will pause automatically (in single player mode only) when you'll move the mouse out of XBill window. General game hardness depends on the value of level, of course, it will affect the number of computers being guarded, number of evil servants for Wingdows [TM] distribution and even the existence of wires between the computers. When computers are connected by wire (into the network), infection of any computer from this connected cluster will cause the infection off all the peers in a while. Any such infection being currently distributed through the wire can be cleaned by using the water from the bucket in the upper left corner of the playing area.
For a more red-blooded view of the game please read its original manual page, please.