Several years ago I wrote a TGA Thumbnail plugin for Windows 2000 / XP. I skipped having a Vista machine and just recently upgraded to Windows7 64bit. I thought that now that I finally have a 64bit machine to do development on I’d try upgrading the plugin to work there.
I started researching how to do it and found out Microsoft added this subsystem they call the Windows Imaging Component. It basically lets developers make plugins to both read and / or write any image format, plug it into Windows and then any app which uses that system can start loading and/or saving those formats. The cool thing is Vista and Windows7 use it for both thumbnails AND for viewing images.
I did a little searching and someone has already written a TGA codec plugin which you can find here. FastPictureViewer WIC RAW Codec Pack. That’s awesome! Saved me the trouble. 
Too bad it costs money so it’s useless to 99% of the people who are looking for a free alternative. :/
Also, Installing DirectX SDK (the developers package) allows TGA thumbnails and many other types. Also, is free, legitimate, and liscenced safe by Microsoft instead of some random 3rd party.
** Download link below **
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=3021d52b-514e-41d3-ad02-438a3ba730ba