Vista Windows Defender updates lockup

I have two separate laptops (different models) that both are having the same issue. Not every windows defender definition updates done thru windows update will fail to install. If I start the install and the problem occurs when I go to shut down the PC it will just hang and never shut down. The only way I get the install to go is to manually go to Microsoft's web site and install the update from there which works fine. No other updates have this issue. This has been occurring for months and is getting very frustrating so any help would be appreciated. - Thanks