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That's a puzzle, to begin with. If it only happens on one computer, though, that it slows down that much, then you have either
A. a defect on that one computer, or
B. a bad port on the switch or router that one computer is connected to?? not likely. Easily determined by swapping cables, one port to the other and vice-versa, with another PC on the same switch or router box. And re-testing.
So it's most probably a problem on the computer. Please give me all the basic details about the affected Gateway computer, and I will advise - or ask for further details - etc.
For instance: what's the CPU speed, how much RAM memory (not hard disk or virtual memory) is on the Gateway, and also, what Ethernet network adapter does it use? a 10/100 type, or something else?