Unfortunately Windows XP and preior versions of Windows doesn't support PNGs as system / other icons... VISTA should/will support SVG/PNG/ICO all...
In XP you need to convert them to ICO...
With iColorFolder tool you have you can change most of folder icons in Windows... but problem with that tool is that is copies the applied icon inside the folder... well tht's good if you like to use separate icons for each folder etc...
For systemwide icon replacemnt, you can always use tools like TuneUp Styler2 / IconWorkshop / Microangelo / IconPackager etc... but all are shareware.
There's something called ICO2PNG / PNG2ICO, which is freeware alternative for converting PNG2ICO and back... ... it's a tiny DOS EXE which hell lot of commandline options... It was bundled with GANT BEAT icons...
Google around for it...
If i find, i'll [edit] this post and add a link to it...
