I think the XCOPY command as above should do the trick. try it out on a test user & system.
/D only copies files over if source file is newer
/H copies hidden and or system files
/I If destination does not exist and copying more than one file, assumes that destination MUST be a directory.
/R Overwrites read only files.
/V Verifies each new file
> Nul If the file isn't there copy it over

Also should your line, if not exists "%systemroot%\system32\myfile.dll" read if not exists "%WINDIR%\system32\myfile.dll" Give it a go.

Regards

------------------
Don Davidson
Network Engineer
http://www.insight-media.co.uk