Do you get to start it in Safe Mode?
If so, can you then do the MSCONFIG thing and restart it, with everything turned off?
Does it crash in that mode?
If Yes to all of those, start again in Safe Mode, do the MSCONFIG thing, go to the "General" tab, and untick all the boxes marked "Process..." and "Load..."
Then try again.
Otherwise, I suspect either a damaged Windows installation, or a virus.