Here's the problem I think you're both encountering:
1) You're using a protocol called POP3 to access your email messages.
2) Your email client (Outlook, Thunderbird, Windows Live Mail, etc.) is configured to leave old messages on the email server.
3) One computer comes along and downloads the mail, which is left on the server. Another computer is used to access the mail, which is then downloaded on that machine. The process repeats for each computer.
4) Because the email is now stored on each computer (and not on the internet server, as is the case if you use the webmail interface in your browser), deletions, sent items, etc. are not reflected across computers.
I suggest that you try one of the following:
1) Use the webmail interface to access your email within your browser.
2) Configure your email to use the IMAP protocol instead of POP3. IMAP will leave messages on the server and will sync all email data across computers. Note, however, that IMAP is not supported by every email provider.