Windows update has been taking hours--which leads me to believe it is an unusually large file or there is something else wrong. How can I stop the computer without causing damage to the hard drive?

Just cancel the update.

open the control panel, click on security center and click on the automatic updates icon. There you can turn off the update. It is probably SP3 trying to load up on your PC. NOW, What you can do is to set it to notify me but don't automatically download or install them. If you don't want SP3 (I myself never use it as it caused many problems with printers, connections, etc in the past...I just use SP2) then follow the rest. Now when you see the little shield near the clock saying updates are available choose the Custom setting. Then uncheck SP3 and it will ask you if you want to omit that item from further updates....put a checkmark there. Now you can reset updates so they load up automatically. Hope that helps Some HELP in posting on Computing.net plus free progs and instructions Cheers

Just hit Ctrl-Alt-Del to bring up Windows Task Manager, find wuauclt.exe on the list, right click on it & select "End Process". After doing that, you can change the settings as detailed above.