Interesting situation.  Had a Dell D820 laptop connecting to a Windows Server VPN (PPTP).  Added a Dell E6500 laptop to the network and it would not connect to the VPN or it would connect and randomly disconnect within minutes of connecting (mostly VPN error 619, sometimes VPN error 800).  Updating the router firmware and drivers on new laptop did not solve the problem.  New laptop and VPN worked fine on other networks.

Router - LinkSys WRT54GS v5

Modem - Speedstream 5100

In the initial setup that had worked for several years, the modem was doing the PPPOE authentication to ATT DSL.  After this problem occurred, we put the modem into bridge mode and setup the router to do the PPPOE authentication.  VOILA.  This solved the problem and now all computers on the LAN can connect to the VPN.

To Make these changes:

Step 1: Put the modem in Bridge Mode:  Connect to the DSL modem directly and reset the configuration to factory defaults.  To get to the configuration page of the modem goto 192.168.0.1 (or whatever address the modem shows on the bottom) from a web browser and click the advanced button on the left.  Then click on the Reset Modem button below.  You will be asked for your modem access code which is a yellow sticker on the bottom of the modem.  Once the modem has been reset, close the browser window and reinstall the router.

Step 2: Setup PPPOE on router:  With the router connected, open a web browser and goto 192.168.1.1 (or whatever the address of your router is - use IPCONFIG and use the Gateway address if unsure) and enter your password to access the router, (by default, the password is "admin" and the User ID is left blank). Once in the router, on the Setup page, select PPPoE as the login type and enter your DSL UserID and password. Set the Time Zone setting and click Save Changes. 

Step 3: Setup your wireless configuration and security as you normally would.

Try your VPN Connection.