Hi,
It is impossible to update Windows Defender on XP SP3 if Avast shields are activated. I tried it both with Avast 2015 and latest version. Windows Defender is already added to the exclusions list. A manually update is only possible when all shields are disabled for a while.
Thanks in advance.
It is not any good, replace with Malwarebytes …
Windows Defender doesn’t come with Windows XP.
http://blog.kaspersky.com/multiple-antivirus-programs-bad-idea/
That’s right, you’d need to install it separately.
Anyway, I strongly suggest to follow Pondus’ advice.
I already have Malwarebytes running. 
I know it. I had installed it years ago.
Maybe I just wanted to report a non-working function in Avast. 
Thanks anyway.
Remove WD on XP, you don’t need it.
Or at the very least disable WD.
It is not Microsoft Security Essentials. It is the pure old Windows Defender. I like the Software Explorer function.
By the way, I didn’t get any problems with WD definiton updates until I upgraded to the Avast 2015. It was impossible to keep my old Avast version. There were problems installing virus definitions or getting Avast to run properly.
You need to decide what to use.
avast or Windows Defender.
Both is a dead end street.
Remove the one you don’t want to use completely.
See the link in reply #2
In fact, he can use it on XP, but it’s plainly useless.
Even more, as he also has MBAM installed…