I have an outlook 2002 PST file. This old format has problems like a 2 GB size limit. I wanted to convert to a more modern PST format (unicode) which doesn't have this issue.
MS's site says there's no direct conversion, but you can create a new unicode PST, then export all the messages from the old ansi PST into it. So I followed the steps on this page: --> http://office.microsoft.com/en-us/outlook-help/convert-a-non-unicode-data-file-pst-to-a-unicode-data-file-pst-HP001038351.aspx
The problem is this doesn't seem to truly merge or import the messages. Outlook now shows two inboxes, and I have two separate PSTs in C:\Documents and Settings\user_id\Local Settings\Application Data\Microsoft\Outlook ...it seems to be keeping track of the both PSTs, with the new one having no messages (and no size limit problems) and the old one having all the messages I wanted to keep (but also the size limit).
How can I get my old messages into the newly-made PST that doesn't have a size limit?
edit: The program linked in the accepted answer kept crashing with a runtime error (terminated unusual way blah blah blah)... both on a windows7 and XP machine. An alternate freeware program worked ok, with some minor hassles - http://www.winpst.com/Ansi_PST_to_Unicode.html
After converting with it, you can replace an existing outlook.pst with the converted file. But it names your personal folder (top level of your mailbox) something like "made with WinPST" and you can't rename it. And you get a mysterious duplicate personal folder that has identical inboxes, subfolders, etc. (new messages show up in both inboxes). Outlook's data files section shows only one PST (plus archive.pst which can be ignored). To delete the duplicate personal folder, I had to do some convoluted steps. Back up your existing converted PST if you haven't already. Create a new data file (control panel --> mail --> data files --> add --> choose the newer format --> give it a name --> click OK). Set outlook to send new messages to this data file (control panel --> mail --> view or change existing email accounts --> change "deliver new mail to" so that it goes to your newly made data file). Finally, run outlook, go into tools --> options --> mail setup --> data files --> click on the outlook.pst (not the new one we just made) and click remove/delete. Yeah, we're nuking the file you successfully converted. You backed it up right? Somehow, rather than deleting all your messages etc., they stay and you delete only the duplicate personal folder. Close and reopen outlook to confirm.