|is it under warranty? that's your best bet.|
otherwise, i'd start with Safe Mode (tap F8 during boot to see this option), and see if you can log in without it crashing that way. If YES, then the issue is most likely a driver issue. Look in the Device Manager (Right click My Computer, Manage) to see if any devices have a yellow triangle or red X, indicating an issue. update any recently added drivers from manufacturer's website, or uninstall completely.
Are there any entries in the Event Logs? (Computer Management, Event Viewer, and look in the Application and System logs for yellow or red). Look through any recent warnings and errors for hardware or software names suggesting an issue, and uninstall or unplug the culprit.
bad sectors on the hard drive itself can also cause instability, and can usually be repaired by running the checkdisk utility from a command line (chkdsk /r /x)
if we're out of warranty, open the side of the case. if there's dust-bunnies the size of your fist, get a can of compressed air and use it to blow out the inside of your case. Just make sure you hold the can upright so as not to spray chemicals all over your hardware. Overheating can cause stability issues, as well as permanent damage to hardware.