As posted on the Outlook groups at least once a day:
On Old Computer:
Close Outlook.
Search for, and copy the pst file. Default name and location are:
C:\Documents and Settings\{Your User Name here}\Local Settings\Application
Data\Microsoft\Outlook\Outlook.pst. (You may have to enable Hidden and
System files in your search)

On the New computer:
Copy the pst file to your HDD. Do NOT overwrite any existing file. Remove
any read-only attribute. (Right-Click-Properties).
Open Outlook.
Do File-Open-Outlook Data File and navigate to where you copied the file.

You then have two alternatives.
1. Drag and drop the data from the old file to the new
or,
2. If you have a large amount of data then you can set the old file as the
Default Delivery location and Outlook will just carry on using that file.
(Control Panel-Mail-Data File)

more info here:

http://www.slipstick.com/config/backup.htm
http://www.howto-outlook.com/Howto/backupandrestore.htm
http://office.microsoft.com/en-us/assistance/HA010771141033.aspx

HTH