Do you have file sharing turned on?
If not go to Control Panel and launch Network Setup Wizard. Go thru the wizard and configure it as you prefer and remember to turn file sharing on.
If you can't find Network Setup Wizard, you have Control Panel's view configured differently than mine, if that's the case then you have to just try to find it.
BUT I wouldn't recommend having file sharing on while connected to internet and I don't remember how Hamachi's file sharing system works.
And BTW if you want to play GTA IV via Hamachi, you have to check that Hamachi is set up as default internet connection in Windows Live (needs registry modification).