Glued Computer
Don't know if you've fixed this yet, but i thought i would add my two-penneth, just in case
If you've checked and there are no processes which seem to be hogging your system think again about heat...Intel chips have a heat sensor in them which slows the chip down if it gets too hot so as to prevent damage to themselves. To the end-user this is preceived as a dog-slow machine. A good way of testing this is to play a game (such as a flight-sim, or something which works your PC really hard) if the game slows down really early on then you probably have a heat problem, most likely a chip fan which isn't spinning. If you can play the game for 10-20 minutes without a hitch then your problem most likely isn't heat related 'cos the chip would be up to full-stress and overheating well before then.
Another reason could be your antivirus doing some sort of background scanning. If that's scouring your hard disk then you'll also get pants performance. Check the settings. Is your antivirus software fully updated? some trojans will dominate your system sending data onto the internet after a short while, that can slow both your PC and your net connection down.
Not so sure about the following....but your problems with sending emails *might* be that the packets of networking information might be having trouble getting back to your PC. If your "in" connection isn't also your "out" connection the machine might just sit about waiting for packets confirming that data is reaching its destination for as long as any timeouts last (possibly indefinitely). If the data coming back has to come via another network, it might just not do it in time. This would be especially true if each connection has a sperate IP address - the packets might not have the rights to get back to the other connection.
Hope that may be of some help.