I just gave this answer a minute ago so I feel like a one-trick pony saying this but...
If your internet connection is suffering AND your computer hardware is being used up then I suggest ensuring its not a program running in the background. Of course those problems could be two separate issues but if they aren't, I have some thoughts.
Try running msconfig from the start menu and disable all the programs in the startup tab. You can also review the services tab (you will want to hide all Microsoft services) and disable those too. Restart your computer and test the connection or application.
A similar way to do this is to use task manager to check the processes running and by viewing the file location (and/or using google), determining whether it is a system-critical process and ending it if it is not. Without restarting your computer, test the connection or application with the only those essential processes running.
This second test can be fairly time consuming - my computer currently has 78 processes running - but it also slightly more "virus-proof" if a virus is the program causing the problems.
From my experience, some viruses have to a way to recheck themselves in msconfig.
You could also install TCPView (free from Microsoft) to see a list of all of your tcp and udp connections. If you see anything that doesn't belong there you could take appropriate action.
This all assumes something is present on your computer doing something you don't want it to do. If there is something absent, that's a whole different story.