Go to Programs -> Admin Tools -> Routing and Remote Access. Select your server by right clicking and choose Configure and Enable Routing and Remote Access. The wizard will guide you through the configuration.
Next, right click on ports and select properties. Right click on the VPN ports that you need. You need one port per connection you want an outside computer to make. (I suggest PPTP ports unless you want to set up certificates for L2TP)
Then, right click Remote access policies and create a policy that is appropriate. The default allows everyone to connect all the time if they have Dial in enabled.
To make sure the user trying to dial in has Dial In enabled, Go to the user's properties, click on the Dial In tab and click Allow Access in the Dial In and VPN field.
Finally, on the client computer (assuming it is W2K), right click My Network Places, go to properties, select make a new connection, choose VPN, and enter the server's information... and BINGO you are done!
When connecting, remember to use the username and password for the user you gave Dial In access to.
(If Win98 client, you need to have VPN support installed (a Windows component). This can be done under Add/Remove Programs. Once/If you have VPN support, go to Dial Up Networking, Make a new connection. Enter an appropriate name (I suggest putting "VPN" in the name), select the VPN adapter, and enter the server name/IP address. BINGO... you are done.
Note: Win98 uses VTP (vs. PPTP or L2TP) This may be useful if you restrict dialin access according to tunnelling type in Routing and Remote Access)