if you try to use bitlocker on your c dirve, you can get a message like this:

what you need to do to make it work is to open (use run ) gpedit.msc

go to  Computer Configuration \ Administrative Templates \ Windows Components \ Bit Locker Drive Encryption \ Operating System Drives and double click on Require additional authentication at start up and enable it.

now trype to encrypt you data with bitlocker and it should work just fine.