I have seen this behavior on at least one Windows 98 machine that had the following configuration:

1. multiple user login accounts
2. multiple user email accounts (one for each login.)
3. mailbox (.PST) files moved from the default location via email client

TaskGhost had trouble locating the .PST file even though it was logged into a user account and the mail client (outlook) was running fine in that account.

I speculate that when I moved my .PST file from within the Outlook client, Outlook stored that information in the "Microsoft Outlook" default profile and not my personal profile.
So, when operating from a client other than Outlook (TaskGhost), the new location of the .PST file needed to be set.

Windows prompted me to locate the .PST file. Once located, all subsequent MAIL access will be fine (even after rebooting.)