I guess I'm a bit of a command line junkie. However, for anyone slightly familiar with a command line, XBMCCmd provides a mechanism for exploiting the power of XBMC from your PC.
For example create a batch file, called xplay.bat, with the contents similar to:
and you will be able to type something like:
It is also useful for writing test scripts.
However, if you want a Windows interface to XBMC, have a look at XBMCControl which is also on SourceForge. Among lots of other functionality it can play media with a link copied to the clipboard, e.g. right click on an mp3 stored on your PC's hard disk, chose Copy and then XBMC will start playing it (if XBMCControl is configured to turn this function on).