Is now available for download and supports the RAW files (PEF only) of various Pentax DSLRs from *ist D to K-x (and for many cameras of other brands as well):-
http://www.microsoft.com/download/en/details.aspx?id=26829
Go get it for the update if you are using Win7 or Vista (SP2 only) and you just want to have a quick preview of your RAW pictures at the Windows Explorer. Enjoy!
Btw, the latest Pentax K-5 and K-r are not officially supported, though. And neither the 645D is mentioned.
On the other hand, Pentax has their own RAW codec that supports K-5, K-r and 645D, but than all the 64-bit Windwows are not supported:-
http://www.pentax.jp/english/support/digital/rawcodec_win.html