Server assigned IP problems.

Here at work we occasionally have a problem where users (both my bosses laptops in perticular)can’t get on the network. I’ll get on their computers and assign an IP, like 10.0.0.100 or some such and that will work for awhile. Then they’ll go out town, come back and it starts over again. The server is a Win2003 SBS machine. I can’t find a root cause for this and the pres of the company wants something more than a bandaid fix, like I use now. Thanks.