On boot, start tapping the F8 button, this should give you the option to enter Safe Mode.
Once in Safe Mode, if it hasn't froze, then you probably have defaulty hardware or drivers or a virus that is crashing the system.
Click the start button, the run, and type msconfig. disable everything on startup and all non-microsoft services. You can check the event viewer in admin tools that may lead you to the culprit.
In addition, check Device Manager, also accesible through the Control Panel/ Admin to ensure know exclamation marks are present for any of the devices.