Have you tried to use the PC without the router? Most of the router ports have auto config enabled and when they get out of sync you lose connection. Try to hardcode the port speed and duplex mode on your PC and router. Most routers are set for autonegotiate and by default XP is as well so nail those suckers up and the problem will go away
Most routers cannot get out of autonegotiate mode but if you nail up the PC side the router SHOULD auto negotiate to the speed you set on your pc. If the router cannot the link will not come up but I am guessing that the negotiation problem is the issue...
Remember work in layers...physical, datalink, network....
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