Rebooting into Windows after I removed it

So I put Windows XP on my iMac for a day until I realized my Windows disc was only good for two computers and had already been used. Anyway, I used the Boot camp utility to remove the partition, but for some reason I boot into what would be XP. It's just a black screen that says, in the Windows boot font, "Cannot boot from drive, insert disc to boot" ...or something along those lines.

So I have to power off and hold down alt/option to boot into OS X. Does anyone know how to get rid of this?

 

Wow, thank you. That's so simple it's embarassing