Blank sreen but laptop is running inthe back ground.

SOURCE: Will not boot to Safe Mode

Try to boot from the Resource CD by selecting the <F12> key and select boot from CD/DVD drive. If you tried to run the "Diagnostics" option from the <F12> boot menu and it went to command prompt, there might be an issue with your hard drive or the "Diagnostics" partition is corrupted or missing. If all tests pass using the Resource CD, then your OS is probably hosed. As for the 9 LED light, that just indicates that your numeric keypad is enabled.

SOURCE: Vista boots to blank screen. Mouse pointer moves. Nothing else

graphic card issue. very bad, very expensive to repair most likely a new motherboard.

SOURCE: Dell latitude d600 screen goes black during boot

ok after you have booted in safe mode do a restore to a date just before the last day it worked properly and try that restore is found here

start/programs/accessories/system tools/system restore

hope this helps

SOURCE: unmountable boot volume

hi,

-this can be a hardware problem or a software problem
-run diagnostics first on the computer
-turn off computer; while the computer is off, press and hold the "Fn" key then press power at the same time to initialize the test
-if you find error message like 0142 - this is a hard drive failure; contact your manufacturer for replacing your hard drive
-if no error is found, it is a corrupted Windows; you need to reinstall Windows, ensure you consult local technician to back-up files before installation.

cheers!

SOURCE: i have a dell inspiron 6400...will not boot..tried

Windows is corrupted you will have to use your Dell Restore Cd to reinstall Windows.The drivers if you have the driver cd will have to be reinstalled as well.
Good Luck