I'm trying to setup vpn on my linux box and get this error when connecting from a Win2k PC :

Error 734 : The PPP link control protocol was terminated.

This error appears right after Registering your computer on the network. So it does authenticate.

How can I fix this.

According to MS:
734

The PPP link control protocol was terminated. There are several possible causes. Try the following:
•

Your Internet service provider might have experienced a problem. Contact your Internet service provider.
•

The security protocols on the server to which you are trying to connect are incompatible with your security settings. In Network Connections, right-click the connection you are using. Click Properties, and click the Security tab. In Validate my identity as follows, select Allow unsecured password.
•

LCP extensions are not supported. In Network Connections, right-click the connection you are using. Click Properties, and click the Networking tab. Click Settings, and clear the Enable LCP extension check box.

Post your options.pptpd file.

How did you install poptop? Did you patch the kernel?
What distribution / version are you running?