"Microsoft Photo Info allows photographers to add, change and delete common "metadata" properties for digital photographs from inside Windows Explorer. When installed, a new "Photo Info" item appears on the context menu for files selected in Windows Explorer. To use, simply select one or more image files, right-click and choose "Photo Info" to open the Photo Info properties editor. You can edit metadata for files individually, or all together as a batch. Photo Info reads and writes metadata in IPTC and XMP formats (depending on file type). It also provides enhanced "hover tips" and additional sort properties for digital photographs in Windows Explorer. Microsoft Photo Info runs on 32-bit versions of Windows XP (with Service Pack 2 or later) and the Windows Vista operating systems."
Ok, along with Paint .NET, this needs to be part of what you install on every machine now. I have pasted a side by side comparison of what the two different apps look like. On the right is the standard details tab on the properties of an image file. On the left is Photo Info 1.0 which has a details tab that resembles the right, but there are a number of other tabs that you can use to tag an image or batches of images. Once tagged, Windows can then search for those tags which is a cool feature IMHO. Two things I would ask for in this app are - 1) the ability to add new types of attributes that I want to tag my images with, and 2) some sort of predictive tagging where the software would scan my images and try and autotag the images as much as it could.