It is not possible to help you anymore. Steam won't help you with this, since it is your own fault for not using security software, like anti-virus or even a firewall. Steam is not responsible for this error, so there is no hope for you. The only thing, that could happen is, that steam gives the user, who received your stuff a ban, but even that could be impossible, since that account could be just a trade partner and not the script kiddy, who hacked your account.
A little tip for the future: Format C: your computer, install windows from a clean DVD and the first thing after installing windows should be a good antivirus program like Avast, Kaspersky or any other good anti-virus application!
A personal firewall would be good, too!