When i try to install windows defender i get this message:

service 'windows defender' (windefend) failed to start. Verify that you have sufficient privelages to start system services.
I have checked forums but none of their solutions have worked so far.
thanks very much for your help

First check to see if the Windows Defender program appears in the add/remove programs under the Control Panel. If it does, uninstall it. Also install the Windows installer clean up utility from here:
http://support.microsoft.com/kb/290301
It will show up in the start menu, run it and see if defender is there, if it is remove it. Then try re-installing it.