When starting a Windows Vista computer you get a message saying that Windows Defender cannot be started and error code 0x800106ba is shown.
After much web searching and trying many options one thing I found worked for me.
Go to Control Panel then select System and Maintenance and then clicking Administrative Tools then finally select Services.
I scrolled down to Windows Defender service and to my surprise it was deactivated and stopped.
To fix this I double clicked that service to open it, then chose the option to start automatic on start up, then I was able to click on the Start service button.
I then clicked on Apply and Restarted the computer ,windows Defender started up fine.
Change your current antivirus to Panda Antivirus Pro 2010 and forget these problems.
Free trial period too :)