Let's try several things, just in case:
1) Open up your list of processes (CTRL+ALT+DEL --> Processes tab) and try to search for psclient.exe. If you find it, select it and click on "End Process," and try running PS again.
2) Does this happen with only one of your characters, or multiple? If you only have one, can you try creating another and attempt logging in?
3) What are your system specs like? If you right-click on your "My Computer" icon and click on Properties, you'll see your processor type and speed and RAM amount. To see your graphics card and driver version, type dxdiag into the Windows Run command and take a look at the Display tab.
4) Have you tried uninstalling completely (making sure no folders remain) and reinstalling one more time?