Unable to view PDFs and/or JPEGS

I have several people in the office that are unable to view JPEGs, GIFs and PDFS from Windows. All of the PCs are running Win XP Pro, Dell PC’s 2.4GHz or higher. No special software is loaded to view these files, just using the defaults. All we get is a box with a square, circle and a triangle and the text comes through fine. It doesn’t matter if the email comes from Outlook 2002, Outlook Express, e-mol and various other email programs.
I have used Adware to check for other problems and not finding anything. NAV 2004 is up to date with Virus definitions. Is there something else I may be missing, or does it require a reload of XP. Please advise. Thx…RFM