I found a great tool to use together with pnotepad: "Character Set Converter" by Benjamin Kalytta. This tool lets you do several character set conversions. I use it with the following configuration (go to Tools -> Options -> Tools):
Name: Convert Encoding
Command: [Path to]/charsc.exe
Parameters: "%d%f" /scs=%? /dcs=%? /replace
Shortcut: STRG + ALT + E (or whatever you like)
Using this configuration, it will ask you for a source and a destination character set and then convert the currently opened file. For available encodings, refer to the manual. Some times one needs to reload the file before seeing the changes. You can use my script at http://pnotepad.org/forums/topic/267 to rapidly reload the current file.
The tool is available at http://www.kalytta.com/tools.php