Why does that occur? The laptop is running Windows 7 and has had new memory sticks installed. I also ran a check on the harddrive and there were no known issues.
Go to Start -> Administrative Tools -> Event Viewer -> and you want to look at Windows Logs -> System. If your disk is causing trouble, you'll see a bunch of events named "disk" and saying that there are errors in reading or writing. A failing hard drive can cause what you are experiencing, but there are also many other causes.
Other things that could cause this off the top of my head are antivirus programs that are trying to verify executables with an online service but can't reach the service for some reason, and network drives that aren't reachable.