Jon,
I myself actually do use IrfanView. Quite good, isn't it? And free!
Unfortunately, here at the office, it's not an approved app, and we're stuck with the XP Picture and Fax Viewer. I searched the MS page for info but couldn't find anything--however, I found the following in the install notes for a scanner:
(paraphrase)
"This scanner produces images in TIF 6.0 format, capable of storing color images, and capable of storing multipage images in a single file. While the default picture viewer is unable to display TIF 6.0 format images, a patch is available for NT 4.0 to enable the viewer to view TIF 6.0 files."
So I figured that if NT 4.0 has a patch... surely XP should have one available to view these files; but so far, no go.