Check that you have no psclient.exe still stuck in your processes whenever you quit. What dilemma do you have when it comes to reinstalling? That option has a tendency to fix some weird problems every now and then, for weird reasons. It's recommended that you always keep your downloaded install file saved away.
Can you check your system specs for us just to be sure? If you right-click on My Computer and look at the properties, you'll see things like the CPU and RAM amount. To see your video card, right-click anywhere on your desktop, choose Properties, go to the Settings tab, and look under Display.
I realize that this is all extra work, but it never hurts to have the exact specs on hand when dealing with technical issues.