Can't Make PDF file extension stick in XP

I cannot get XP to recognize the PDF file extension in my user account.

I've removed and reloaded the Acrobat reader without any luck.

When I check the file types and associations in the folder options, the pdf extension does not appear. When I try to add it and associate Adobe Reader 9.1 with it, it doesn't stick. (i.e. - I go through the steps, shut the window down, and then come back and pdf is still not on the list of extensions)

If I define an alternate extension such as 'xxx', associate Adobe Reader with it and then open a *.pdf file that has been renamed to *.xxx , Reader opens just fine.

When I switch over to an administrator account, everything works fine.

In the administrator account, all pdf files have the familiar Adobe icon, in the user account all pdfs files have a non-descript icon.

Any thoughts on what the problem might be here?

Thanks for any direction you could provide