why this one pc..cannot be pinged nor appear in the network yet it can see and ping other machines in the network.please help.
If that is the case then it is almost certainly some third party software present on the problem machine that is resulting in your problem.
So determine what is different in respect to software on that one machine, third party firewall software like Comodo or ZoneAlarm, third party Security Suites like Norton, McAfee, AVG, etc., even remnants of such software that remains from an incomplete uninstall can result in these types of problems. Uninstall any such program using its' own uninstaller or the programs and features, uninstall applet in the control panel and follow that up with...