Another possible solution is for those that participated in the StarCraft 2 Beta.
Note: that the following steps require that you modify your registry, you should only attempt this if you are sure you know what you are doing.
Blizzard cannot be held responsible for any damaged you may cause modifying your registry
Click on Start then choose Run.
In the Run box, type the word "regedit" (without the quotes) and press OK. This will launch the Registry Editor.
Click on the + icon beside the "HKey_Current_User" folder.
Under that, click on the + icon beside the "Software" folder.
In the folders that appear under "Software", then open "Blizzard Entertainment", and Select the "Battle.net" folder and delete it.
Click on the - icon beside the "HKey_Current_User" folder to collapse it as well.
Now that both "Battle.net" folders are gone, close the Registry Editor, restart the game, and single player again.
