All right I won't mention a PRAM reset....
Since you haven't stated exactly what you have tried I will suggest booting from your install DVD 1...
Insert the DVD1 into the disk drive and hold down the 'C' key while restarting...
after you select your language...like you were going to reinstall the OS (but don't) go to the menu bar at the top of the screen and select 'Disk Utility' and from there choose your 'MacIntosh HD' from the left column and then the 'First Aid' tab and 'Repair Disk'...
Also you might try holding down the 'Shift' key to boot into 'Safe Mode'....see if it will boot there...
This link speaks of several things to look at to free up some drive space..
http://www.thexlab.com/faqs/freeingspace.html