Web Site Connectivity

We have several computers (all XP) connected to the 614+ router. Two of the computers have access problems with several web sites (one being Microsoft Update), the rest of the computers don't. This happens whether these two computers are wired or wireless into the 614+. Both computers have access when connected directly to the DSL modem.The wireless card is D-Link 520+, the wired card is a D-Link DFE-530TX+ We've tried lowering the MTU and to no avail. Any suggestions?

Thanks much.