Hello Danilov,
BitLocker is a new feature of Windows Vista Ultimate, Windows Vista Enterprise for client computers and Windows Server 2008 for servers. It could only work after installing these Windows OS.
So, I am afraid that you cannot use BitLocker to encrypt your disk before installing Windows Vista. For more information, please refer to:
BitLocker Drive Encryption
http://technet.microsoft.com/en-us/windows/aa905065.aspx
If you'd like to know something about the Full Disk Encryption subject, the following hyperlink might be helpful.
http://www.lsi.com/campaigns/listenin/index.html?cid=40063
Cheers!