Cannot connect to internet. XP SP2.

Can ping gateway, can arp. Can detect conflict in ip. Reloaded ie6, reloaded drivers for nic. Reloaded Protocols, checked cable, jack etc.. There are two computers out of 500 that have this problem. They are both dell optiplex. They are both in same lab. Have switched lan cables for computers. They just will not connect. Any ideals?
Thanks
Bill