Test the RAM, and make sure you use the diagnostic program on the drivers and utility disc. The built-in diagnostics on Dell systems tend to suck. One time I spent a couple of days chasing down what turned out to be bad RAM. I didn't suspect RAM, because I ran Dell's onboard diagnostics, which allegedly checked the RAM, and it came out fine. I run the memory tester on their diagnostic disc, and it fails in seconds.
Anyway, once upon a time, not too long ago a Dell system came through my office. Similar symptoms to yours. System would POST, then reboot. Turned out to be bad RAM.
One other thing, is to turn on SMART in the BIOS. Dell likes to leave that off by default. This is a predictive hard drive failure system. It usually gives you at least some warning if the drive is going, so you can back things up.
Run the diagnostics off the drivers and utilities disc or make your own from the download on Dell's support site. First run the memory test, then run the express test. If you can't even boot a CD, then pull one of the RAM modules and see if that helps. If not, pull the other one, and replace the original so you've tested each RAM module individually. Be sure to inspect them for a blown chip. People sometimes say that the smoke is let out of them, because that's kind of what it looks like when it happens. If a chip blew, there'll be a white "smokey" residue.
Once you've done all that, and you still haven't found anything, come back and we'll go from there.
I have a Dell Latitude E6400 running Windows 7.
Today my computer decided it no longer wanted to load Windows. I was surfing the web fine and then it suddenly locked up. After I had to force a restart, I get that Dell logo going as it loads, and then after that instead of loading the operating system it will just stay at a black screen. If I stare at the black screen long enough the computer will restart itself and I will repeat the same process.
I tried to go into safe mode. Kept pressing F8, and the safe mode boot option would never load. It would just keep shooting pass the "Dell" screen and into my black screen as if it never registered my F8. This has never happened to me before; no idea why I can't even get the safe mode option to load up.
I ran diagnostics, and it says the hard drive is still functioning.
Any idea what is going on here?