Try testing the hard drive and the memory. To test the hard drive go to Computer and right-click on the C drive, select Properties, tools, check now. Set a disk check on the next start up. Let the check finish. To test the memory go here: http://www.memtest.org/
Download version 5.01 and burn it to a CD or DVD. Boot the laptop with this disk and run Memtest86 for at least 7 passes. No errors are allowed...