Sharing a .PST file between two Outlook programs is generally not recommended, as they're really not meant to be shared. If you only have a POP connection, you'll probably be better setting your Outlook to leave messages on server (you should have it delete the messages after a certain number of days though). This will allow you to retrieve your incoming mail with both computers. The only thing about this setup is your Sent mail wont sync up, so you wont be able to see a message that you sent from the desktop on the laptop, and vice versa.

If your email provider supports IMAP, you may want to consider going this route instead. With an IMAP connection, your mail is essentially stored on the server and both computers will sync the mail folders with that server copy. This allows you to sync other mail folders beyond just the inbox.

If you do decide to try sharing your .PST file over the network, you'll only be able to use it on one computer at a time. In other words, to connect to the mailbox on your Laptop, you'll need to make sure that you've closed Outlook on the desktop first.

The .PST file format is essentially the same between 2007 and 2010, so you shouldn't have any problems there.

Hope this helps.

-SW