Hi Paul,
Manifest files are XML text files that accompany many kinds of executable
files - eg EXE and DLL files. The software developer will create a
manifest, to regulate how the associated program file will behave.
Manifests are kind of like INI files, except each Manifest usually
controls just one program.
You should NOT delete these files! Doing so could cause your applications,
or Windows itself, to stop working properly.
You can find more background information about Manifest files here:
http://msdn.microsoft.com/en-us/library/aa375365(VS.85).aspx
A normal Windows installation may have hundreds of files with the
*.manifest extension.
(Although, if the files you're seeing reported by Windows Defender are
*not* XML files in manifest format; but just some other kind of file
called "Manifest", then I don't know what they are!).
Hope this helps,
Andrew