What was the mail client on your old pc that you have exported from? (Outlook, Outlook Express, WLM ?)
What is the file extension? (dbx, pst ?)
In some cases you need to have Outlook installed if it was on the old pc.
Also as the 'inbox' is a default folder of the mail client I feel you would have been better to have put your emails into another folder of your own creation before exporting them.