This one came up recently and I thought I would post a quick note about it.
I had a client that needed to move email from one PC to a new PC. This client had over 1GB of email in Outlook.

I had the set up the users email account on the new pc using their correct username and login info. I then simply copied the pst files from the old PC to the new one using a 4GB USB flash drive.

The files were located in C:documents and settingsappdatalocalmicrosoftoutlook*.pst (where is your windows login name)

The files can them be moved to the same location on the new PC.