Boot into Safemode!
See if you can boot in safemode.
Ture PC on and press F8 for at least 5seconds...
Once boot screens loads up, use 1st option..by the way use the down/up Page Key's to scroll to this option Safe Mode and enter.
PC should load if all is ok at this stage..
Now if you can load up windows in safe mode close down and start PC AS NORMAL to see if you still have this problem of the crashing of your system
Option2. is as before but you may want to try the option of Load last known working profile....settings something to this effect and see how you go from there..
1.Have you installed anything new since the last time your system was working?
2.Was you using the net the last your PC was working? (You may have a virus)