Your paint registry files have probably been messed up. Luckily, it's an easy fix.
Try this:
Open the registry editor (Start-->run, type regedit)
Delete this key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\Curre nt Version\Applets\Paint
Now restart Paint, and it should work.
For more information and detailed instructions, look here:
http://support.microsoft.com/default...;en-us;q185501
hope this helps,
~psi42