The simplest method is manual backup at regular intervals on any media such as USB pen drive. Normally, these files found at C:\Documents and Settings\Administrator\Local Settings\Application Data\Microsoft\Outlook. (You should enable ‘show hidden files and folders’ from Windows explorer Tools menu in order to view the files). When restoring it you need to copy the files to the very same location from where you copied it earlier.
Microsoft has developed an add-on for Outlook (2003/2007) that lets you backup and restores your emails quite easily. And it reminds you at regular intervals for backup. You can get more information and download from here. After installation you can see the backup option from File menu. Just click on it to backup your mails or restore it. Click the Option to change the reminder.
Another way of email backup/restore in Outlook is by using its ‘Import and Export’ feature. Click on File menu and select ‘Import and Export’ and select ‘Export to a file’. Select ‘Comma Separated Values (DOS) and click next. Next select what you want to export such as Inbox, contacts, etc and save this, for example, on a USB pen drive. To import the CSV file, just select ‘Import and Export’ and select ‘Import from other Programs and file’ and follow the instruction.