.Pst files can be quite large, so it’s a good idea to make sure your backup location has plenty of room—and that you allow lots of time for an email backup to occur. After you’ve done that, just follow these steps to back up your Outlook content:

1. Open Outlook.
2. In Outlook 2010:
   Click the File tab, and in Backstage view, click Open, and then click Import.
3. In Outlook 2007:
   Click File, and then click Import and Export.
4. In the Choose an action to perform list, click Export to a File, and then click Next.
5. In the Create a file of type list, click Outlook Data File (.pst) in Outlook 2010 or Personal Folder (.pst) in Outlook 2007, and then click Next.
   
6. In the Select the folder to export from list, click the folder you want to export from, such as Inbox or Sent Items, and then click Next.
7. Browse to and select the location where you want to save the file. Remember, backups should be placed somewhere other than the original location of the source file. For example, if your source file is on your computer’s hard drive, you’ll want to save your backup file to an external source, like a CD or an external hard drive.
8. Choose the default setting Replace Duplicates with Items Exported.
9. Click Finish.
10. At any time, you can restore your file by importing it into Outlook.Note: If you want to just view or access something in your exported .pst file without importing it back into Outlook, you can simply open the .pst file.In Outlook 2010:
    Click the File tab and, in Backstage view, click Open, and then click Open Outlook Data File.In Outlook 2007:
    Click File, point to Open, and then click Outlook Data File.

In closing
See how quick and easy it is to protect yourself and your data from permanent loss? Backing up your data might take you a couple of extra minutes a few times a month, but you’ll be glad you took that time if an emergency ever happens.

Article by: http://www.microsoft.com/athome/setup/backupdata.aspx