Bios password protection

I have my system on drive C: and also I have all my important folders on drive D:. I want to lock this drive with a password. Any advice?

Go get TrueCrypt.

Feature-List (taken from the site):

• Creates a virtual encrypted disk within a file and mounts it as a real disk.
• Encrypts an entire partition or storage device such as USB flash drive or hard drive.
• Encrypts a partition or drive where Windows is installed (pre-boot authentication).
• Encryption is automatic, real-time (on-the-fly) and transparent.
• Parallelization and pipelining allow data to be read and written as fast as if the drive was not encrypted.
• Provides plausible deniability, in case an adversary forces you to reveal the password: Hidden volume (steganography) and hidden operating system.
• Encryption algorithms: AES-256, Serpent, and Twofish. Mode of operation: XTS.

There's no easy solution for "securing a harddisk or a single partition with a password". You could, of course use a BIOS password, but this won't stand a chance if the attacker is in front of the PC (CMOS reset jumper f.e.).

Some harddisks do offer the functionality of a password, but as far as I know, the BIOS will have to support it, too. There are also multiple problems with it, forgetting the password will turn the harddisk into a nice brick, and in most cases you can crack it as easy as the BIOS password.

Do you want BIOS password protection or do you want file system protection? If you want the latter, update the title of the question and then check out TrueCrypt. If it's the former, update the body of your question and then reboot your PC and check out the BIOS menu.