Even a bad driver can be the problem for this.
Try to boot in safe mode with f8 button when you power up.
Or try the VGA only mode to boot, if this work then you have a faulty driver and you should de-install it from the software screen.
After this, boot up normally and install the right driver from the website of ATI, NVIDIA or INTEL.
If you dont know with brand is installed: install this program to find out:http://www.astra32.com/