Search for file poclbm and delete. In windows, click start and type poclbm into search box. Delete and try running guiminer again 
note: this will delete any mining info you've saved, so for slush, you'll need stratum.bitcoin.cz:3333 and your log-in info. Good luck
note: this will delete any mining info you've saved, so for slush, you'll need stratum.bitcoin.cz:3333 and your log-in info. Good luck
I just tried it, it didn't work.
If you are on windows7, the file 'poclbm' is in the directory C:/Users/YourUsernameOnTheComputer/AppData/Roaming/poclbm.
It contains the information saved using 'File, save settings' about your various workers as well as the window position when saved. You can delete the file (thereby losing the saved setups, but also getting rid of any strange window positioning), and this is a safer option that trying to edit the file directly! It's easy to setup the information again for Slush!
Edit: assuming the window appears in a normal position when you restart it!
Note: Use the 'close' option on any gui-miner icons in the righthand tray thingy as you don't want to be restoring the mystery missing instance of the program!
End Edit