VPN Error

Hi We have sevaral PC’s here ( HP Vectra 800’s) and compaq laptops and desktops using Windows 2000 Pro. The problem here is when you connect to a VPN connection over the internet or if you connect to the internet over the phone line for ExampleNetzero. As soon as you disconnect from that connection the pc blue screens and reboots. I installed windows 98 on one machine and VPN works good and doesn’t blue screen on the pc when you disconnect. Our California office is having the same problem. I have tried to install service packs, called tech support to compaq and Hp they couldn’t come up with an answer. any suggestions or help would be great,
thanks for you time,
Larry