Repairing your .pst file depends on which Windows OS you're running.
You should think about either repairing your .pst file (for XP have a look
here) or deleting it and simply syncing the data from yourX51. Alternatively, if you follow a good PC (not ppc) backup protocol, simply restore the information from your last known good archive.
A word of warning,
before deleting files from Outlook, remember that your X51 will not have copies of all of your email to sync "back" to your PC and you stand a good chance of loosing data! Make a
backup of your .pst file
FIRST
In Vista, your .pst will be located at C:\Users\
user name\AppData\Local\Microsoft\Outlook (where
user name is your id on the PC).
In XP, your .pst file will be located at C:\Documents And Settings\user name