Go to Handango and search for "tune" and/or "guitar." You'll find a few great tuning applications.
There is a great free tuning application which is no longer supported by its developer, but you can still download it here:
http://download.com.com/3001-2176-10193778.html
The neat thing about this freebie (besides the price) is that you can use it to tune a variety of instruments. I'm not sure if it's compatible with Pocket PC 2003, though, because it doesn't seem to work on my hp2215.