Same problem on Windows 7, 64bit
Got this problem in my last reinstall (or at least I noticed it). I'm also using vista x64.
solved it by enterig component services: c:\windows\System32\comexp.msc
then go to component services > computers > my computer > dcom config
find mediamonkey entry, right click propierties
go to security tab and add your user to "launch and activation" and "configuration" permissions.
after doing this, no more errors logged.
My concern is with network restrictions that may be applied by adding a User to the "launch and activation" and "configuration".
The simple way to get to Component Services, type comexp.msc in the run command press enter.
I solved the issue by going to 'Component Services > computers > My Computer > DCOM Config > Mediamonkey (Right Click) > Properties > Security Tab. Then select 'Use Default' for all three options.