System Rebooting after ISDN Modem Config

I m using SANTIS communicator External Siemens ISDN Modem but problem is when its been configured my system rebooted, its not happend rapidly but after 3-4 hours system restarted. I takes following step to find out the problem:

1- I scanned system but there is no any Virus
found.
2- I changed system from Compaq DeskPro to IBM
compatable.
3- I changed my OS from W2K Server to
Professional.
4- I applied ever option of Siemens ISDN Utilities