For several gigabytes of emails? You cannot be serious!Assuming you can't just redownload them from a server, then the easiest way is probably to manually migrate the emails rather than convert the MBOX format to PST or whatever.
- Create folders in Finder (maybe on your Desktop) that correspond to each folder in your email account (Inbox, Sent, and so on).
- Drag the emails from Entourage to the corresponding folders. You may need to do these in batches since it might have trouble with similarly named emails and very small emails.
- Get the folders onto the PC.
- Drag the contents of each folder into the corresponding folders in Outlook or Outlook Express.
I hope that helps. Do you want to move contacts over too?