Your file is corrupt.
Norton Disk Doctor used to do a pretty good job at fixing these types of errors.
This is not an internal corruption within the PST file itself moreover, it's a currupt DOS file system. Normally, SCANDISK or CHKDSK would remedy this however, I'm always reticent about running high-level repair tools which may have a tendency to truncate a corrupt file without attempting to actually retrieve fragments of otherwise lost data which. Again, Norton used to include a tool called Norton Disk Editor (NDE) which I found particularly helpful on several occasions however, some level of expertise is required to make the best use of these tools.
As a suggestion to the above, it's a good idea to create a shortcut to SCANPST.EXE and run it every so often.
Another overlooked Microsoft utility free with XP is NTBACKUP. You can set this up to do a weekly (or even daily) scheduled backup to avoid the same problems in the future.