I installed V 10.0.2 64-bit when it was released. Since then it worked, except for the bugs. I used it earlier today, no problems. However this evening, opening Mathematica lasted a very long time of "formatting notebook contents". Creating a new document, opening document and everything that relates to "Mathematica.exe" is delayed at least 1-2 seconds.
I disabled the welcome screen which made it a bit better though.
I also have an old V 220.127.116.11, which exhibits the same behavior when I open the program, and shows the welcome screen. Opening/Saving etc is not affected in this version.
I tried restart, reinstall v10, and move the files stored in AppData/Roaming, and AppData/Local, but nothing changes.
After the reinstall, when I press "enter activation key" I firstly gets an error: then it opens a window saying "activation succeded", but the "enter activation key" thing is still in the menu.