BitLocker is a Windows Vista security feature that encrypts an entire hard disk (technically, a volume) to protect your data if someone steals your entire computer. If you see, “The drive configuration is unsuitable for BitLocker Drive Encryption. To use BitLocker, please re-partition your hard drive according to the BitLocker requirements.”, it means you need two partitions (a partition is a smaller section of your hard disk). As shown in the figure above, BitLocker needs a small, 1.5GB “active” partition to store the Windows Boot Manager, which basically decrypts your BitLocker-protected partition so Windows can start. The main partition, your C: drive, is the BitLocker encrypted one with your personal files, the paging file, and everything that needs to be encrypted.

Instructions after the jump.

Microsoft promises to release a tool to repartition your hard disk for BitLocker. In the meantime, follow these high-level steps to repartition your disk:

1. BACKUP YOUR COMPUTER. Your messing with your disk here, and if things go wrong, it won’t start. Don’t take chances. After the backup, ensure that your C: drive has at least 1.5 GB free space. If necessary, use the Disk Cleanup Wizard to free space.
2. Open the Computer Management console by clicking Start, right-clicking Computer, and then clicking Manage.
3. Right-click the C:\ drive and then click Shrink Volume. Configure the Shrink dialog to reduce the size of the C:\ drive by at least 1.5 GB. Then, click Shrink.
4. After shrinking the C:\ drive, right-click the new, empty volume in the Disk Management snap-in, and then click New simple volume.
5. Use the New Simple Volume wizard to format the volume with NTFS.
6. The formatting might take several minutes. Afterwards, you will have the two partitions necessary to use BitLocker. However, the smaller partition must be marked as active. Right-click it, and then click Mark partition as active.
7. After changing the active partition, Windows will no longer start. Restart the computer from the Windows Vista DVD (the computer must be configured to start from CD/DVD), and start setup. When prompted, click Repair your computer.

8. Startup Repair will automatically detect a problem. When prompted, click Repair and restart, and then restart your computer from the Windows Vista DVD. If Startup Repair does not automatically detect a problem, continue to the next step.
9. When prompted, click Repair Your Computer. This time, manually launch Startup Repair from the System Recovery Options. Startup Repair will detect that the Active Partition does not have the boot manager installed, and copy the required files to the active partition. Remove the Windows Vista DVD and restart your computer normally.

Now, the computer has a small boot partition separate from the system partition, which meets the disk partitioning requirements for BitLocker.