disconnecting from lan

Check the network settings and make sure the PC is on the same subnet and that the DNS settings are all right.

Inspect the wall outlet and the flylead, checking for any damage, crunched sockets or terminators. Replace the cable and try again.

Check the corresponding port in the switch to see if the light is on.

Try to connecting another computer to the same wall outlet, maybe using a longer flylead, and then check the port in the switch.

Try connecting this PC to another wall outlet that you know surely is in working order.

If the problem can be narrowed down to the PC, you may need to reseat or replace the network card. Otherwise, get a cabling technician to fix the cable between the wall outlet and the switch.

Troubleshooting is basically narrowing down the cause by elimination.

sandip if you have done all those that toivo recommended you , and you are sure all those are ok AND still problem persists then it is the good old NETBIOS over tcp/ip problem. Probable soluitons: upgrade to XP , create a new network connection from scrtch on the problematic client , and finally enable Netbios over tcp/ip in your advanced network tcp settings.