Windows 2000 SP3 hang

In installing Windows 2000 SP3 on one W2K Professional machine, the install hangs in terms of the indicator bar being only 70% done. SP3 appears in Add/Remove Programs, but the install wizard stops short of allowing one to click on Finish.

Ending task is the only way to get out of the SP3 install, then the software can be uninstalled from Add/Remove programs successfully and then prompt for reboot.

What is the install of this SP conflicting with?