A computer is like a heater with fans and some lighting. In an average computer, at least in an average computer I build, there's 4 fans (PSU, CPU, GPU and enclosure). Put them at 2W each (and it's often less than that), you waste 8W to rotate the fans. The HD LED and Power LED need maybe 1 watt together, let's say 1W each to be sure. The only other power used not to produce heat is the rotational mecanism of the hard drive and of the optical drive. Put them at 8W each (and I'm very generous since the optical drive is idling +99% of the time). So that's 8W+2W+16W wasted in mecanical work and lighting. All the rest is ultimately heat dissipation. If I have a system that sip 100W, about ¾ is heat dissipation (probably more like 85%-90% because I was very cautious in my math). I'm pretty sure a 75W heater with a good airflow is better at heating a room than a 100W floor heater that only irradiate heat and therefore concentrates the heat near the heating device. Heat production of the former will be lesser, but its distribution will be much better.
Now, a floor heater with a fan next to it would be better than a computer, but that's normally not the case. But in any case, I prefer a heater that improves my FAH stats than a dumb resistor smelling funny the first few times you use it in Fall.