Assuming you are using XP, go to Start -> Control Panel -> Mail, this will open the Mail Setup dialog box the top button is E-mail Accounts. You can add more email accounts that are also setup on your server.
Another option would be to create the new email message then change the From to a different name, this would let you change on the fly, while still retaining your default account the rest of the time, i.e you don't have to keep switching back and forth.
If you don't see From in your Outlook 2007 message, like the options tab of the message then click Show From button in the Fields section. I use this to change to a generic "Support" email when replying to some customers, and then the rest of the time Outlook will default to my normal username email.