If the virus scan doesn't work, Try this:

First, try booting into safe mode (press F8 during boot up). Your PC should be running completely normally. If it isn't, you have some problem with your OS

Assuming your PC is running fine, boot into safe mode with networking. If your computer still works normally (should be able to use internet now), All of your networking components / drivers are fine.

Once you get to this point, if everything is running normally, open up the run window and type "msconfig" into it.

This window will have tabs for various diferent system commands, but the two that you are interested in are startup and services.

In the services tab, check the box that says "hide all microsoft services". Now select "Disable all", and press OK. You will be prompted to restart your computer, go ahead and do so.

Assuming that everything is still looking good after this, The problem is with one of your system services. From here it is process of elimination. Start systematicly enabling services until your machine starts fucking up again. Once you hit that service, you know what is causing the problem. You will have to restart each time before any of these modifications take effect.

Assuming that when you disable all non-microsoft services your computer is still slow, the problem is with one of your startup programs. Again, process of elimination. Disable services and continue restarting till you find the one that hangs the system, and bingo, there's the problem.

You may want to try disabling / enabling 5-10 services at a time. It will mean a lot less restarts on your part.

And if it's still broken, you can be pretty damn sure that it's a hardware problem.