Try going into

- (On Windows 7/8) Control Panel -> Network and Internet -> Network Connections
- (On Windows XP) Control Panel > Network and Internet Connections

You should see a bunch of icons for your different network connections.

1. Go into the menu at the top (if it is hidden, press the <kbd>Alt</kbd> key to show it). This is the menu that says File, Edit, View etc.
2. There should be an *'Advanced'* dropdown. Click that, and select *Advanced Settings*.
3. Using the up and down buttons, move your Hamachi network connection to the top of the list.

Then try again. This can sometimes fix issues with Hamachi not working correctly.