Windows ? -> reboot
First step is to reboot your computer.
Then you should check your disk(s) for any errors, AND correct them, if there'r any.
What Antivirus are you using ? You might want disable that while strugling with this problem.
DON'T use the Internet while antivirus is disabled.
Oh yes, are you THE administrator when doing all these steps ?
Don't have windows 7 myself, but vista,
One more thing: is EVRYTHING Calibre related, programs, library etc. located on the c: drive ?
Sorry that I don't have the magic solution for you.
Hopefully someone else can help you, but you might to give us all some more info.