This exact same problem has been an issue for at least 5 years - since W2K - on several models of Dell laptops. I have personally experienced the problem and know of at least 15 other very technical consultants who have had the same issue.
Unfortunately the solution we came up with was not a very friendly one - as you may guess it required a reinstall. However, after the reinstall the problem did not reoccur.
My personal view is that there is some process that forms part of a standard Dell initial install that causes this problem on the occasional laptop.