Hi all - just had the same problems - with a Dell Studio 1555. It begun with the keys 1, 2, 3, 4, 7, 8, 9 and 0 stopping working, and I saw plenty of cases of the same thing happening to others. I thought it might be a software problem, and tried the diagnostics - but in the end, it simply was the obvious - a malfunctioning keyboard - being dell, there are plenty of replacement keyboards available on online auction sites - I got one for under £9 delivered in a couple of days, and it worked perfectly. I'm no computer whizz, but it was straightforward to swap in the new keyboard using a +tive screwdriver and an online guide
All the best
Doug