Are you using the correct VPN server name? The server administrator provides this name.
Is the Internet / network connection to the server functioning properly? You should ping
the server if not sure.
Is your network firewall
blocking VPN connections? Port 1723 must be opened and Protocol 47 must be enabled in the firewall configuration settings
Does the server have too many clients already connected (VPN server connection limit exceeded)? The server administrator can check this, but it is not a common problem
Also make sure PPTP is enabled on your VPN Connection. and router.
One thing I can tell you about Vista - if you're using OneCare, the OneCare
firewall blocks outbound VPN by default. You need to enable it in the
advanced settings. I'm not sure about the built-in Vista firewall - that
may be the case with that one as well.
also are you running microsoft virtual PC 2007, if so uninstall it and then try becasue vp2007 sometimes cause problems with PPTP
Hope this helps