My Static IP changes to DHCP

I recently found I am not connected to the Internet. When I checked my network properties, my static IP, gateway, and DNS server IP addresses were gone. My network settings had reverted to DHCP. My static IP address is reserved on the DHCP server. So now I input the IP manually and in about 8 minutes I have no Internet access, so I go to networking properties and it is on DHCP again. I have scanned my system with TrendMicro OfficeScan 5.6 and no viruses or worms. I have Windows 2000 SP4. Can you help? Thank you,