Well, you are getting the "Startup Disk Is Full" message because... your startup disk is full. You have put enough data and programs on your machine's main hard drive that there is no room left. This gives your OS a problem, because it needs some hard drive space available to operate. If you doubleclick the icon of your hard drive on the desktop, at the bottom of the finder window it will say how many Gb or Mb are available. What is that number?
The solution is you need to delete some things from your hard drive
#1 is Empty your Trash.
#2 go to
www.Versiontracker.com search for and download DeLocalizer. This will remove unneeded foreign language resources from your OSX, saving about 1 Gb of space.
#3 buy yourself an external Firewire hard drive that you can store your documents, music, movies, whatever it is you are filling the drive up with, and also backup your data to.
# Again at Versiontracker.com, look for a utility like CacheOutX that will delete caches and temporary files.
Couple of other things.
You *might* have a runaway process that is creating huge log files. At versiontracker.com look for a utility called WhatSize, which will list the largest files on your drive for you.
Caution: DO NOT start deleting or renaming files and folders from your hard drive unless you know they are items that YOU created. Apple's OS creates specific files and folders (including hidden folders) which MUST remain in the same place with the same name for the OS to run. Don't mess with them.