Although this topic did solve my problem with PC waking up right after switching it into sleep mode it helped me to find out what was wrong.
As you can see on the Delicieuxz's screenshot the network card wakes the machine up from sleep. In my case it was antivirus process (run as service)
Owner: [SERVICE] \Device\HarddiskVolume2\Program Files\Bitdefender\Bitdefender 2012\vsserv.exe (VSSERV)
So what helped me? I've opened "Power options", selected the plan I wanted to fix (Balanced power plan) and then clicked -> Change the plan settings -> Change advanced power plan settings and from the next dialog window "Power options" I've expanded the branch Sleep, then expanded Allow wake timers, choose Disable.
After that I've found MS article on that ;] Sleep and hibernation: frequently asked questions