FWIW It is probably not a VMware Fusion issue per se. In other words it probably just a Windows issue regardless of the fact it's running in a Virtual Machine or not and as such you should probably turn to Microsoft Resources and or Goggle for help with your non-descriptive Windows Update issue.
Note that this is of course assuming the Guest OS has basic Internet/Network Connectivity.
Otherwise you need to provide additional information to help troubleshoot the issue, like any error messages, etc.
If you feel it is other then something to do just directly with Windows and more a VMware Fusion issue then... To help figure out what is what, the best way to provide comprehensive diagnostic information is to use the "Collect Support Information" command from the VMware Fusion (menu bar)> Help > Collect Support Information and then attach the .tgz file it created on your Desktop to a reply post.