Hello blcArmadillo,
Firstly - welcome to
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1) What security software is running on these PCs?
2) Is the Dell switch 10 or 100MBPS?
3) Are all the network cards set to 100MBPS Full-Duplex?
4) What encryption settings are set on the router?
5) Is it a 802.11G or B router that you are using?
Please answer these questions so that we can diagnose your problem.