Dells have a boot menu available on their splash screen,
before the OS boots. F12, maybe F10, is the key (F1 or
F2 gets you the bios). Did you try booting up via the Dell
boot menu?

You might also try running "memtest" from a Linux CD
as a 'burn-in.' I'm partial to Mepis for their memtest option.
I used it on some refurbs I recently put together. One PC
would stop on memtest no matter what RAM chips I ran.
Turned out all the memory tested fine once I tried a different
CPU. Obviously it may not be possible to swap out a CPU
on a laptop, but memtest can be a good indicator if you've
got a hardware issue. I routinely run memtest on any RAM
upgrade I do.