We're going to need additional information, including system specs (age of computer, processor, RAM, etc), any changes you made to your system recently (software or hardware), if there was a recent power surge, and if you've kept up with internet security software (antivirus and such).

Beyond that, we need to know how far the boot process gets, as there are several steps. For instance, does it crash while the Windows boot screen is displayed and the bar is going across the bottom, the logon screen where you sign in, OR the desktop, where it shows your icons, the start menu, etc?

My first suggestion would be to try to boot into Safe Mode by repeatedly pressing F8 during boot. It will take you to a list of options...use the arrow keys and enter to select Safe Mode without networking and see if it preceeds without indecent.

Hope this helps and let us know more.
John