You might be able to fix this with a pram reset (command option p r at boot until the boot chime sounds thrice), but I've had this problem before, and I seem to recall fixing it by reinstalling the OS and/or wiping the hard drive. If I were you, and I had this problem, and reinstalling didn't work, and I had an extra hard drive, I would back up my data and erase/install. If that still doesn't work, say so, and maybe we'll come up with something else.

You should be able to hold down the option key and pick the boot up system you want. If this is something you don't want to happen, go to System Preferences - startup disk and select the operating system you want. If I wanted to get clean, I'd back up using Time Machine to an external drive. Then do as Sterling suggests. I would also try booting into Safe Mode by holding down the Shift key when stating up.

thank you guys i will try and see what happens. I have an external but how do i back it up?

You can clone your drive to the external via the command line (that's what you're booting into), type this:

sudo rsync -avrogx / /Volumes/[Backup Volume]

Where instead of typing [Backup Volume] type the name of your backup, if it's more than one word, say it's called "Backup Volume, instead of typing:

sudo rsync -avrogx / /Volumes/Backup Volume

type instead:

sudo rsync -avrogx / /Volumes/Backup\ Volume

If your volume is just one word, say "Backup" then that's easy, just type

sudo rsync -avrogx / /Volumes/Backup

Should be a piece of cake.

i replaced the hard drive quit easy. that was the problem!!