It appears that Windows 2000 SP4 has an end of support date at July 13, 2010. Windows Update should be working correctly until that point.
Keep in mind, since there have been updates to the 'Windows Update' service and 'Background Intelligent Transfer', you may need to manually install SP4 and SP4 Security Roll-Up 1 before you can connect to Windows Update for the rest of the updates to bring you to current.
Once I manually applied SP4 and Rollup, my old laptop running Win2k Pro (P2/333mhz/192MB RAM) connected to Windows Update fine, as recently as a few weeks ago.