I've had issues in the past where users were having timeout issues with SharePoint sites on SBS 2003.
Particularly users with custom form sites or lists that require a lot of data entry will notice that after 15 minutes they are disconnected.
So to extend this default you need to make the following change:
1. Click Start, point to All Programs, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
2. Double-click Web Sites.
3. Right-click the virtual server that you are referring to, and then click Properties.
5. On the Web Site tab, in the Connection timeout box, increase the amount of seconds to a more appropriate number.
6. Click OK.
This should resolve the problem.