>Hi everybody,
>
>On my husband's laptop we're getting 'Operating system not found" when we try to boot the computer. However, this problem is not always reproducible. Sometimes it boots and it works for 20 or more minutes before displaying this message.
>
>Before turning this laptop to professionals do you have some ideas what can be at fault here?

Check the event log for disk-related errors e.g. Event ID 7 - "bad block". This is usually an indication that the hard drive is failing.

Be very sure to back up your critical files, to USB, network etc. Consider doing an image backup to an external USB hard drive. That way, if the hard drive fails, you can replace it with another, restore the from the image backup and you're right back where you were.

Image backups are also good tests of drive integrity. If an image backup won't complete due to a disk error it's another indication the drive is failing. Often, CHKDSK won't fix these types of problems, even with the surface scan option. One utility I can highly recommend for mapping out bad or weak sectors, and maximizing retrieval of data from those, is Spinrite: http://www.grc.com/spinrite.htm