ESG -
Try the MS website
here.
There's one article in particular that might be relevant:
here.
Also, make sure that XP is patched to SP2 with all updates and security patches - ditto for IE / Outlook.
It might be worth disabling the AV software (stop the services, set to manual, reboot) to see if the problem persists - if it doesn't it may be AV related. Don't forget to reset the AV service to Auto and start it again!