How to tell/monitor if my Outlook email is hijacked?

I am using Outlook 2003 / WinXP Pro SP1.
I have NAV 2003 with virus definitions up-to-date.
I run SpyBot Search & Destroy once a week.

I connect to the web via Netgear ethernet router with built-in WAP to DSL modem.

My connection is wired, not wireless.

I am seeing mail in my inbox, which is coming from valid sites (like the NY Times) and is sent in reply to something coming from me.

Problem is: I never sent the outgoing message.

How can I tell whether my email was hijacked, and if so, how can I stop it?
Alternatively: how can I tell whether someone is simply impersonating my email and sending from a different location?

thanks.

I have seen the spammers use the RE: thingy. Look at the headers on the mail and see if it is just from a person(spammer) or the NYT. Look at the mail and try to figure out waht they are repling to. If it is fishy then contact the NYT and find out.
Use mail encryption, and you will have to worry that much less.

Thanks for the feedback. If I use mail encryption, doesn't everyone I email with need the same encryption?

Here is a basic link to mail encryption.....
I thought it was good read if you want to secure your email.
http://www.dtek.chalmers.se/~d97jorn/pgp/