background tab missing under display properties on windows 2000

There might very well be a policy enabled to prevent users from changing this. If you do not have access to the policy, then you can change the following registry key:
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\
System]. Look for NoDispBackgroundPage. If the value is set to 1, then you cannot access these properties. Set it to 0 and you will be able to access that tab.