Trojan Horses

we need more to give you good answer. what error message if any? what anti-virus software? what os?
if you have up to date anti-virus software and definitions and have run your virus scan with XP's system restore function turned off, or, in Safe Mode, and you don't get any report of viruses, then you are most likely clean.
what symptom has you wondering if you have a trojan horse? perhaps you might want to download and run a spyware remover such as Ad-Aware from www.lavasoftusa.com
the emails returned that you have not sent are because others with your email address in their address books have become infected. not you necessarily

Along with the above you might wanna run stinger.exe. Will check for viruses

Try running ad-aware on your system which you can download from downloads.com for free. I think you your e-mail address is being spoofed which means your not really sending the e-mails someone making it look like you is using your e-mail address as a reply address to send out viruses. There isn't much you can do about that but just to make sure you don't have a trojan try running your virus protection and scan your system then run ad-aware and get rid of what it finds. Make sure and update ad-aware before running to get the latest detection tools available. Hope this helps. I use this tool on my network.

try the new version of Spy-bot S&D also, if you are running ME or XP disable the system restore before you run your antivirus. spy-bot has a nice selection of tools in their freeware. (like the shredder)

Turn off system restore if it is used in your os.

Download, update, and run the following.

Your Antivirus Software

Spybot:
http://tinyurl.com/ziar

Ad-Aware:
http://tinyurl.com/tek5

CWshredder:
http://tinyurl.com/2bzef
Or
http://tinyurl.com/2k642
Look for the file in English CWShredder.exe

Hijack This:
http://mjc1.com/mirror/hjt/

Hijack This is useful in that it shows what is currently loading on startup. You must know what is good and what is bad. Once you check it and fix it is gone. So be sure.

Run Your Antivirus again

I have had to boot into safe mode and run these.

Good Luck Lee

Go to http://www.windowsecurity.com/trojanscan/ and run their on-line Trojan scanner. I'd also run on-line virus checkers. One can be found at www.antivirus.com
Of course, getting rid of them is a different story, and one answer about hijackthis will help you. Be carefull with hijackthis unless you know what you are doing.