Windows Vista Ultimate, Office Outlook 2007

Starting 10/1 I haven't been able to send or receive. My ISP has reviewed
all settings. My Computer Club guru has been here for an hour and can't
determine the cause.

We have moved a PST from my laptop (running Windows XP and Office Outlook
2007) and opened the Data file on the Vista system. It appears to be empty,
yet the PST size is 49MB.

Any Thoughts?