The above is a little, free program for Windows that works on both 32bit and 64bit and can add lots of image previews thumbnails to filetypes (it also adds some other minor features).
Most notably it supports .psd's and .ai's (ai support needs an extra .exe which is available on the above link, too).
The shortfall of the program is by default it also offers filetypes such as png which are already natively supported, but for some reason turns their backgrounds black when they should be transparent or, at least, did for me.
All the filetypes it interacts with are customizable however, so I recommend turning them all off except things such as .psd and .ai
) so I use 