That can be caused by number of things.
1. Corrupted registry.
2. Corrupted files. (system or programs)
3. Dying hardware (HD, RAM, Mobo...)
You should try booting in safe mode (press and hold f8 while booting) and look in your event viewer (in "administrative tools" <- might be that you would have to check the "display administrative tools" in Start/settings/taskbar and start menu/start menu/customize). In the event viewer look for the entries with the time-stamp of your latest failed boot attempt, and look for the errors and/or alerts. If any, please post them.
Also, while you're at it, look in Control panel/system/hardware/device manager, and if any of the listed devices are marked with red cross (disabled) or yellow triangle (not started), please post that too.
Oh, yeah, Your hardware specs, please...
Chaky
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