It seems like everything you want to do in Microsoft Outlook is harder than it should be, but if there’s a will, there’s generally a way to get it done. Here’s a how to guide for backing up your Microsoft Outlook folders and emails in a .pst file. You can scroll down if you’d like to see the how to guide for opening your already backed up .pst files in Microsoft Outlook. This guide applies to XP (and, I believe, Vista – although I haven’t confirmed that):
1. Go to File import and export:
2. Select export to a file:

3. Select “Personal Folder File”

4. Expand the (+) sign where it says “Inbox” and click on the folders you want to back up:
5. Click browse to select where you would like to save the .pst file. Hit “OK” when you’ve picked out the right location, then, click “Finish”

6. Your backup file is now saved! You can now go into your outlook and delete the folders from your inbox (since you have the files backed up).
If you ever need to reopen the files that you backed up, follow these instructions
How to open .pst files
1. Go to File import and export:
2. Select “Import from another program or file” and hit next.
3. Scroll down in the option list and select “Personal Folder File”, then click next

4. Locate the file by browsing to it, and click, “open”. You can leave the options unchanged (see below), and click “next”

5. Highlight the area where you would like the “folder” to appear in your inbox, then click finish.

6. Your folder will be restored, and you’ll now be able to access all those old emails.