Until recently, I could use a dds thumbnail viewer from the NVIDIA developer page to view DDS textures withing Windows Explorer thumbnail viewmode. However, I recently made the switch to 64-bit Windows XP, and that program doesn't work with it. Can anyone recommend an alternative?

Have you tried DXTBmp?

For IrfanView you can download a plug-in to view dds.

http://www.mnwright.btinternet.co.uk/programs/dxtbmp.htm

DXTbmp...works for me. :)

(there is a download link at the bottom of the page)