Someone hacking my steam account when this happens?

[theOden]

Sure makes it look like people are still flying Dover

[Troll2k]

When Steam installs on a machine it takes a snapshot of your computer hardware.(finger print).

If you install Steam on another machine(or have upgraded hardware) it will send you a notice that someone is trying to log on from another machine.They send a code to your registered email.If it is you,just on another or upgraded machine then the original,just enter the code and you are good to go.The code is very time sensitive.

If it is not you then change your password as someone is trying to hack your account.

[von Pilsner]

If you think your steam account is hacked, you should probably change your email password as well because hackers need access to your email to validate their copy of steam (using your credentials).