Hi Hugh,
Depending on what version of Outllok you have (the following is for MS Outlook 2003) then you need to click on
<Tools> <Options> and then click <Security> tab.
Under security zones, there is a button to allow you to change your download options. Obviously a good idea to make sure your Antivirus is good quality and up to date as you are effectively removing a line of defence.
Hope this helps.
F - Wyg.
(I believe I'm right in saying that viruses could only be spread by .jpg files in XP home without service Pack 2)? Someone else will know for sure)