In WinXP, there are certain default right click menu associations on some file types, such as images, audio, video, etc. These can often interfere with your chosen default apps for handling these file types (such as ACDSee or others). Images are a prime example -- no matter what other app you install for manipulating images, the "Preview" option is always the default, which goes and runs the Windows Image and Fax Viewer. I finally found the solution to this. In the registry are the following series of keys:
And so on.
Any entry in the ContextMenuHandler key will override whatever you try and do, there's no easy way of telling it you want a different default option. This leaves us with deleting these keys as a final solution.
As always, back up your registery before doing any changes.