how to change the speed of a fan with only 2 wires?

[ExecutorKevin]

my fan sounds like a hoover, its a 120mm one that I found in one of my old pentium II cases, and when plugged in to the motherboard header its really good, blows a gust of wind and managed to cool my graphics by 6 degrees, but I can't change the speed of it, there is no yellow wire, so I can't monitor the speed, but surely I can increase/decrease the voltage somehow, Speedfan is useless, and the bios has no options on it, and it would be a shame to throw away a perfectly good fan. So are there any programs that can do this or something, as I don't want to buy a controller, thanks!

 

[ExecutorKevin]

Thanks Tealc for your information and time, but I'm hoping for a quick fix and don't want to buy anything, and that molex switch thing looks good, but I don't wan't to be fiddling with anything, and the pulse the 12v thing, I don't know what you mean, I'm sure the yellow wire is only for the speed sensor so could I still change the voltages somehow from the motherboard header?

Thanks