This is not strictly security related but I thought I would post it anyway ...
The following Skype post provides some helpful insight into this but since this has happened more than once to me and I am lazy to do it by hand every time I went ahead and created a quick batch script for it.
All you have to do is create a new file called "cleanup_skype.bat", paste the following code into it and then save the file:
echo "Did you exit skype already? if not, this is your chance!"
echo "Deleting skype data ..."
rmdir /q /s skype
rmdir /q /s skypepm
echo "Done! Try again"
When you are done you just have to exit skype, click on the cleanup_skype.bat file and you should be good to go. Skype will ask you for your credentials next time you open it but after that you should have the call button enabled.
That's all .. happy calling!