The Microsoft Camera Codec Pack 16.0.0652.0621 teaches Vista+ServicePack2 and Windows7 to handle PEF generally. It enables every software which uses Windows Imaging Codecs (WIC) to display pictures saved as PEF (MS Explorer and Viewer, and a range of 3rd party software).
hxxp://www.microsoft.com/download/en/details.aspx?id=26829
(change "xx" to "tt")
It also supports 64 bit Vista and W7, but generally not XP and not (yet?) W8.
It works for PEF formats created with K100D, K100D Super, K10D, K110D, K200D, K20D, K7, K-x, *ist D, *ist DL, *ist DS.
It DOES NOT support PEF files from K5, as this uses some new enhancements of PEF not existing or used up to K7.
As the K01 doesn't offer PEF format anymore, it looks as if Pentax may skip it in the future. That's why I recently stopped using it.