There is no way your PC is heating a room 6x4 meters. Thats a total heat loss of about 2.4kw (working on 100w per square meter which is about average for anything build more than 5-10 years ago). If you live in a new super insulated house then that could drop to about 1.2kw but still I doubt your computer will keep up with a heat loss of that. I would advise buying a thermostatic radiator valve and turning it down a bit.
The only way for the computer to generate heat is by using energy and the only place it gets energy is the PSU. Even if its rated at 1kW the computer is never going to put out 1kW worth of heat. When I mean 1kw of heat I'm talking about a 1kW output radaitor designed to operate at temperatures of around 60-65degress not your computer putting out 25 - 30degrees air from the case. If you wanted to heat a room on a source of 25-30 degrees it would need to be so oversized its not even worth think about.
Turn down your home heating, crack open a window and you'll be fine.
EDIT: For example in my living room which measures 3.8x4m sits the following equipment mostly on all day
My new HTPC machine - 3570k @ 4.7ghz, gtx680, 2hdds, all in a small case so my case temps are highish
42" plasma
AV reciever
Sky HD box
Wii
As it is currently meant to be the summer our home heating is turned off and the room generally feels fine. Room temperature sits about 20 - 22 degrees depending on weather. Our house is a 70's build mid terrace with wall insulation so our heat losses are not that high (party walls of a terrace don't count under heat loss calculations).