Turning off the computer

I have Windows XP on my computer. What is the worst case scenario if, when I want to turn the computer off, I would press the "on" button on the tower, instead of clicking Start, Turn Off Computer, Turn Off? Thank you. abm

 

Worst case...

Data corruption (you do have backups don't you ), which may very rarely cause your computer to be un-bootable.

Normally, if you have to do so then run an error check on your boot disk when restarted even if you're not prompted to do so.

EDIT: Most systems nowadays perform a proper shutdown even if you hit the power button, killing the mains supply though is different.