well, if you can get ANY Windows XP disc (matching her version, pro or HOME) run a repair.
The Null error doesn't seem to be a OS problem but it is more related to a program filling a database with an empty field. XP comes with .Net support wich is basically VB7. I had seen this problems with VB6 and Oracle, but not from a OS itself. VB can integrate with the OS, so watch out.
If you can, look at the event log after producing an error.
Which program are you using to access email? Is it Outlook Express??
Did you try another program??
Netscape7, Outlook
If the error is reported in a Windows style window, it will probably write a log, so there is the best place to look