Before there was BitLocker Drive Preparation Tool, life was hell with Bitlocker. Its only after installation of Vista completed you release that the only way to enable Bitlocker was to format and reinstall. Well, BS or not, those days are gone. Actually you can technically do some tricks using partioning software and copying files in protected mode...
Anyway, with BitLocker Drive Preparation Tool now, all you need to is to run through the wizard and it will merge or split up a 1.5GB free space for Bitlocker. If you dun use TPM, you need 1 more step to edit the Global Policy. After that, reboot and you can enable Bitlocker. Its also wizard based helping you make your keys and print your recovery codes.
So, what are you waiting for? Wait, before rushing in, do a full backup. JIC.
BitLocker Drive Preparation Tool :