Laptop restarts while connecting the internet modem

Hello there,
I am greatly suffering from connectivity & restarting problem. When I start my laptop if my laptop is not connected with my Qubee internet modem then it restarts after connecting the modem. But if modem is connected before starting the computer then it does not restart. I am currently using windows XP. Even I tried with other laptops but there I didn't faced the problem with my modem. So is it the problem with my windows or any other internet related software or virus? Please it will be a great help for me if anyone can explain my problem with specification.
Thanks
