Milo is quite right that Office 2000 .PST files have a hard 2GB limit. Note that this is lifted in Office 2003 and above. Outlook 2010 can have .PST files up to 50GB!
I agree that migrating to OE isn't the optimum solution, but I disagree about the file size - in OE, each folder has it's own .DBX file, whereas in Outlook ALL folders are in a single .PST. Unless all the mail items have been left in the inbox / sent items, it might actually be better!
A better solution is to create additional .PST files and move mail from the default .PST into the new PST - either by folder or groups of folders.
What can I do about Outlook's huge PST?
SD