You didn't change any folder permissions or rename one of your system folders? I'm sure you did but did you try to hit CTRL+ALT+DEL once you got to that screen? Also, how long did you let your computer sit at the boot screen before giving up?
Sometimes updates take a long time to install and make you think the computer is frozen when it's just updating. I would look at the hard drive activity light. If it blinks a lot then Windows is updating. If it blinks once a minute then it may be stuck. Try holding down the shift key when you reboot your computer. Also, if you have any USB devices plugged in, unplug them or turn them off.... other than a keyboard or mouse. You may have a printer for years without any trouble but sometimes that changes and they'll keep your computer from starting normally.