You might want to check your group policy settings for firewall program
exceptions.  The following are supposed to work (according to the help
information):

   %windir%\system32\sessmgr.exe
   %windir%\pchealth\helpctr\binaries\helpsvc.exe
   %windir%\pchealth\helpctr\binaries\helpctr.exe

I'm however, getting the same error when trying to connect to both desktops
and laptops on my LAN, even with the above-mentioned settings!

The event log also reports the following error:

DCOM was unable to communicate with the computer (computername)  using
any of the configured protocols.

A search on the MS Support site turned up nothing to help resolve the
problem, apart from a suggestion to check a certain registry setting, which
turned out to be set correctly on the PCs in question.

Interestingly enough, if a user firsts requests assistance, then Remote
Assistance works.  The problem happens when an administrator tries to offer
Remote Assistance, instead of accepting an invitation from the user.

Regards,
Brian