You have two choices:
Pocket PC 2002
Windows Mobile 2003/SE/WM5

If you are not sure what operating system your PocketPC has, follow these instructions:

Click on "Start Menu" -> "Settings"

Click on "System" tab

Click on "About"

If Windows version is 3.x you have Pocket PC 2002	If Windows version is 4.x you have Windows Mobile 2003