You can't do it in Outlook Express.
The suggestions above delay it a short while - rather than send immediately, it leaves it in the outbox until the next time mail is delivered (either scheduled or manual).
To schedule something for, say, a couple of days/weeks hence is remarkably difficult to do - even though it's a perfectly reasonable requirement.
I have done it with Microsoft Outlook (not Express), which is part of MS Office - it has a delay delivery option - but that was using a Microsoft Exchange Server - I'm not sure whether it would work on Internet mail - it may be a server feature.
You could try www.lettermelater.com - I've never personally used it, but it seems to do what you want (although you can't have attachments).