Is this happening in one file or all files?
If it is only one file that is causing the problem, try copying the contents into a new file and continue using the new one.
If it is all files, try deleting the AIPrefs (Adobe Illustrator Prefs on Mac) file by closing Illustrator and going to:
On Windows – Documents and Settings > User >Application Data > Adobe > Adobe Illustrator CS* Settings (or your version of Illustrator).
You will need to show hidden files in order to see the Application Data folder.
On Mac – Open the Finder and then go to Go > Home > Preferences > Adobe Illustrator CS* settings (or your version of Illustrator).
Once you have deleted it, restart Illustrator and see if it happens again.