I remember similar question was asked before, but

I couldn't find it using the search. So the credit should be deemed to the person who answered. Basically, you can go to Control Panel -> System -> Advanced, then click on "Settings" in Startup and Recovery section. Click "Edit" button beside "To edit the startup options file manually". Remove the deleted OS from the file. Or you can edit boot.ini under c:\ directly. Be certain that you remove the right one, otherwise you won't be able to boot your machine :-).