You said it is an on-board NIC, but I didn't see which model NIC or motherboard.  Also which module 
are you using to run the NIC?   Broadcom Gig NIC or nForce2, other?

Try different switch port, different cable, check driver model and version, try to ping from 
gateway to pc if possible or from another PC to the broken one.  If your switch is set to 
autosensing, try setting your NIC to 100Mb half Duplex.  You could quickly check the cable and 
switch by putting a standard old 3Com NIC in the box that should be autodetected.  I keep several 
of these around for this purpose.

Brian Kelsay