What to do now in a few steps for you:

1. Install Anti-virus (Get Avira Antivir, AVG Antivirus these can co-exist)
2. Install Spybot Search and Destroy and Malwarebytes Anti-Malware
3. Run all these on all your hard disk drives with the most thorough scans you can find.
4. Scans complete, threats removed. Go to 5.

5. Change the password on your e-mail account(s). Go here: http://www.pctools.com/guides/password/ and generate a password and write it on a piece of paper so you remember it. *

6. Register at Steam Support and let them know your account has been hijacked. Include account name and the ORIGINAL e-mail address on the account. Wait for a response.
7. They will ask you to include all sorts of logical information, give them as much as possible. Wait for a response. **
8. You got your account back! Change the password to something else, see step 5.

Now, be careful in the future!

* I recommend you use mixed case and numbers in your password.
** If you don't have access to a scanning device a high quality picture will be sufficient to capture images of your retail box serial codes.

[ZfH] NoExIt