Please follow the steps given below:
1) Pre Installation Procedures to be followed on the Vista Machine
a) Disable User Access Control -
http://technet.microsoft.com/en-us/library/cc709691.aspx
b) Disable Windows Firewall & Disable Windows Defender
To Disable Windows Firewall
Click on Windows Start Button
Click in the Type Search Box
type services.msc
Click OK
Scroll to Windows Firewall
Right Click on Windows Firewall and Click on Properties
Click on "STOP" and change the start up type to Disable.
Click on OK
To Disable Windows Defender
Click in Windows Start Button
RUN
Scroll to Windows Defender
Right Click on Windows Defender and Click on Properties
Close the Services window
Then,
Click in Windows Start Button Again
type msconfig
Click on the Start up TAB
Uncheck Windows Defender box
click OK
Click on Restart to restart the Computer
Now To Install Symantec Endpoint Protection on Windows Vista