I'm guessing a "loopback file" is the same thing as a "disk image"? (E.g. .iso or .img?) If so, then you need two pieces.

First you need a way to access ext2/ext3 filesystems in Windows. This can be accomplished using Ext2 IFS for Windows. See the FAQ regarding ext3 support.

Second, you need a way to mount the disk image (loopback file) as a virtual drive. This can be accomplished using a free tool like Daemon Tools, or a commercial product like Nero ImageDrive. Also, there is an unsupported tool for XP from Micro$oft called Virtual CD Control Panel (which I have never tried).

Once you install support for ext2/ext3 in Windows, you'll probably have to reboot. Then fire up Daemon Tools (for example), select the loopback file, and mount it as a new drive (E: for example). Now you should be able to read files from the new virtual drive.

P.S. Even though Ext2 IFS for Windows supports writing to a ext2/ext3 filesystem, you might not be able to write to a virtual drive (i.e. your loopback file).