I thought I had tried about everything to get CR2 RAW support up and running in Windows XP. I installed the Canon Codec and tried to open the files with the regular Windows Fax and Picture viewer. FAIL!
I installed Microsoft's Powertoy "Microsoft RAW Image Thumbnailer and Viewer for Windows XP" but that failed with a "cannot load image" message. I was at wit's end. Until ...
Finally, I discovered that Microsoft's Live Photo Gallery that is part of Vista and Windows 7 and that does suppot CR2 RAW files was available for Windows XP. Download it here: http://download.live.com/photogallery and then uncheck all the vile software you don't need. Et voila, CR2 RAW support from your explorer in Windows XP.
Whew. That only took me about 2 hours to fix.