Welcome to the Apple Support Communities
Reinstall the original OS X that came with your computer. If it came with DVDs:
1. Insert the Mac OS X DVD and press C key when your Mac starts.
2. Go to Utilities > Disk Utility and erase the drive.
3. Install Mac OS X.
If it didn't come with discs:
1. Press Command, Option and R keys while your Mac is starting
2. Open Disk Utility, erase the disk and install OS X
Thanks for the quick reply. Do I restart holding down just the "option key" or both the "Command Option and R" keys? I've tried "Command and R" but with my wireless keyboard can't seem to get my iMac to restart other than just going back to my normal desktop.
Won't just an erase of my HDD take care of any sensitive files?