Hi malahavock, and a warm welcome to the forums! 🙂
Could be many things, but it's best to start with these two steps, which may fix it also, but it sounds like something is corruted on your Hard Drive...
"Try Disk Utility
1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, *you must select your language first.)*
*Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.*
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume.
6. Click Repair. Disk Utility checks and repairs the disk."
Then see if it'll Safe Boot off the HD & use Disk Utility from there to Repair Permissions, reboot once more.
See if it'll Safe Boot, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, reboot when it finishes.
http://docs.info.apple.com/article.html?artnum=107392
Get back f there are many errors or it doesn't work.