spyware/adware or firewall slowing you down maybe?

>I have a home network connected through a router (SMC brand, wire only) and I usually have 3 PCs connected with rarely more than 2 in use. The 2 systems I use regularly do have a hub between the router and the PCs. It is a cheapo hub supporting 10/100.
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>On all systems the network icon in the Systray reports that the connection is 100.0 Mbps. This suggests to me that the hub is not an issue.
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>When I D/L from a (certain) site I get speeds reported between 60Kbps-45Kbps. Every now and then I will see as much as 100Kbps but that has become rarer and rarer.
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>I called my provider today as my annual payment is due, to learn what's up with it and how I can get what I'm paying for (3 Mbps (translating to approx 2.1 Mbps with overhead)).
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>He pointed me to a speed test site (their own) and I ran the test. I got 1.3Mbps. Same on both regular systems.
>Then, as they are wont to do, he insisted that I hook up directly to the modem and try again.
>I did so and the speed test reported 2.4Mbps!!!
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>So it looks convincingly like it is something at my end.
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>Anyone have a clue where I should look? The cabling is all CAT5 and I'd guess I'd see other symptoms if it was cables.
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>Any iideas appreciated.