I use http://www.irfanview.com this seems to set itself up Ok as the default picture viewer in XP Pro for me

You can change the associated file extensions in the properties of the program, it never fails for me. There is also an option to have one one instance of the program running or as many as you want (ie a new instance launches everytime, leaving the previous one in the background) Hope this helps.

Cheers

Cris.