Could be a lot of things but I'd start with a close look at your isp. Look up a good speed test and see how you measure up.
http://www.google.com/search?q=speed+test
This is the one I use through my isp.
http://www.inetontario.com/speedtest/
If you find you're slower than you think you should be, first step is to shut down computer, router and modem. Keep it down for a full five minutes. Check all wires. Boot your modem first; let it warm up five minutes before you turn anything else on. Then start the router. Let it have a minute or two. Finally, start your computer.
Get a UPS with protection on the network line, not just a battery backup for the power.
If you're still slow, call your isp.
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Secondly, it might not be you at all. If a server is experiencing heavy traffic or a denial of service attack, you can expect what you're describing. Also, if the servers you're after are either hosted by the same company or located in the same area, it's possible the networking is poorly routed which could result in bottle-necks, resulting in the same effect. Nothing you can do but wait it out.
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Could it be the anti virus? Sure, why not. Those programs are so arrogant they'll mess with anything, including your connection settings. The way to tell would be to look up how to manually optimize your connection settings. If you find a lot of differences, and some of them significant, then uninstall (don't just disable) the anti virus, reset the connection manually, and see if it improves.