There's no universal reason for messages to be sent automatically to a junk folder that's valid by default across all possible email clients. The receiver has to set up message rules for that. So all you've got to do to answer that question: check the receivers rules.

On that other computer: make a new Windows XP/Vista account and setup Outlook there. Then disable any outgoing antivirus check you've got (you know, that programs spamming themselves in your mail by adding something like 'This mail was checked virusfree by X' at the bottom) and try again.

Kees