My Toshiba/Windows Vista laptop starts up normal but will not go past the "Welcome" screen.?

I was restarting my computer after an update for my iTunes and I think that one of my family members accidentally turned it off (which might have caused the problem). Also, in Safe Mode, it says that Windows Installer is supposedly not properly installed. And when it does go past the Welcome screen after I do a... show more I was restarting my computer after an update for my iTunes and I think that one of my family members accidentally turned it off (which might have caused the problem). Also, in Safe Mode, it says that Windows Installer is supposedly not properly installed. And when it does go past the Welcome screen after I do a System Restore, it is enormously slow. PLEASE HELP!
Update: iTunes wouldn't reinstall because the Windows Installer doesn't work. Any other suggestions?
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