Laptop hangs on opening attachments

OK the setup is as follows:
Dell D400 laptop
Win XP, Office XP
Up to date A-V etc.

When the user uses a dialup connection from home her system works fine until she tries to open up any attachments from outlook – when that happens the machine hangs and can only be resurrected by killing outlook in task manager.
User is fine when working on the lan.
No obvious processes/apps appear to be conflicting with outlook – event logs don’t give any info on this.
Any ideas?