IP changes by its self

I use Windows 2000 pro SP4 on a Compaq N600c laptop wired to the network and also sometimes with a wireless card. I sometimes notice I can not get to the Internet so I check my IPCONFIG and find my laptop has reverted to DHCP. I lost my static IP address. Then I go back and install all the static IP settings and its fine. I sometimes have to reset them twice a day. What is causing the laptop to loose the static IP addresses? Thank you.