swap ports on router and see if problem stays on the same machine or is the router. If stays, then look at the NIC settings on that machine, could be it is set to 10BPS instead of 100BPS, which would be 10 times slower. If not that then I would suspect settings, viruses etc.
I have a network of 3 computers using DSL and a router. 2 of the computers work fine, the internet connection is very fast. But when I turn on the third computer, it slows down the internet access to a crawl on all 3 computers. Also, he internet works very slowly on this particular computer whether the other two computers are on or not. Any suggestions for fixing this problem would be great! Thanks!