Problems installing Update KB935509: Vista Enterprise

I am having a problem installing KB935509. I am running Windows Enterprise on
my PC. I have tried numerous time to install but no success. To troubleshoot
the problem I booted in safe mode and went into Control Panel/BitLocker Drive
Encryption. Found the following message;
"Your system volume is not configured correctly to allow you to use
BitLocker Drive Encryption. This windows license does not support BitLocker
Drive Encryption. To use BitLocker Drive Encryption please upgrade your
windows license�
Should not BitLocker Drive Encryption be standard with Vista Enterprise or
are there different versions of Vista Enterprise? Thanks in advance.