From Mobileread forum member Alissa:
This is an announcement/advertisement of my small Windows program for Kindle users (including K4PC): Alissa’s MobiHandler.
Once installed, this small program makes your Windows Explorer to handle .azw/.mobi/.prc files better; cover images are shown as thumbnails, and you can see the book titles and authors on the list of files. You can also see the Properties of the books. It helps managing Kindle contents on Windows PC, especially when you download a lot of (probably free) .mobi files from the internet.
It works on Windows XP (SP3) and Windows 7, including 64 bit editions. It should also work on Windows Vista, although I have not tested yet.
And it really does need XP SP3. My crap PC had to roll back SP3 due to it slowing everything down. Nevertheless, I downloaded MobiHandler to try it and got no luck.
On Windows XP SP2 issue: some key features that MobiHandler currently depends on are originally new to Vista and back ported to XP SP3 for compatibility.
And now Amazon also broke the program (accidentally?):
I’ve just noticed that my MobiHandler doesn’t work with recently released Amazon Kindle for PC (K4PC) version 1.5. If you install both my MobiHandler (any version) and K4PC 1.5 in a same PC, MobiHandler doesn’t work.
It appears that Amazon changed the COM ProgID (an internal identifier used by Windows; not usually visible to users) for Kindle for PC, introducing File Association conflict with MobiHandler.
But this looks like something many people can use, so monitor that forum thread for updates.