I just read in a FAQ entry that installing Hamachi and running GPGnet could result in a lot of different errors, and that the general rule of thumb was to not install Hamachi at all.
Well, I am running Hamachi, have been for a long time, and this is just not correct.
It may result in some problems, but this is easily fixed by the following.
Go into "My Network Properties" (Rightclick on your "My Network Places" and choose properties)
Click the "Advanced" menu, and then "Advanced settings"
Here, make sure that your Local Area Network connection (or, whatever you have called it), is placed higher in the list than the Hamachi adapter, and all should be well.
Remember, everytime you update Hamachi, it will be placed at top-priority, so you will have to do this when you update (Its a Windows thing, I have discussed it with the developer, and theres nothing he can do about it, unfortunately)