Same problem here with the clipboard and pasting between applications and the Print Screen. It looks like Photoshop isn't always checking the clipboard, if it has been updated by external applications. This could be fixed with a small registry change.
I've used the registry-fix here on four different PC's running Win XP Pro Sp2 with Photoshop CS3. Everything now works perfectly.
NB! This is the same registry-fix that is provided in your Photoshop CS-package:
Photoshop CS3/Goodies/Optional Plug-Ins/Photoshop Only/Optional Extensions
Look at my page for the registry-key script for both CS, CS2 and CS3:
EXTERNAL PAGE: Explanation and registry-clipboard fix
(edit the script with notepad to see what is added to the registry...) The script is just 131 bytes...
(Chojin: The reason for the "all versions" is a zip, is because all three versions use the same file-name, and needs to be the same file name in order to work. Difference is: