Services that a VPN rely on?

Im trying to set up a VPN from Windows 7 to connect to Windows XP server. I used the Windows client and server and I had it running for like 1 minute, browse the files use the internet through the server, working perfect. All the sudden it stopped now it wouldnt even connect. No clue what is going on. Basically I'm just curious what services a VPN relys on. I might have some off, one being NetBIOS over TCP/IP? Does that one effect it? I am just missing something small because like i said it was working perfect, so I dont know if it has something to do with the authorization, encryption or what, btw Im using PPTP and I do have my ports open (1723)