use computer power supply to run motors
Is it possible to use a computer power supply to control some 12 V motors with my Arduino and these motor controllers?


Yes, you can use a PC power supply as a general 12V supply, but there may be some extra requirements. 

Some early PC supplies required a substantial minimum load on the 5V output to stabilise the supply. I believe this is less common now. If it was a requirement you could "waste" some 5V output energy by connecting a light bulb or resistor or other suitable load. 

PC power supplies may often have coupled output inductors to increase switching noise filtering capability. Noise level and voltage accuracy may suffer using 12V only, but with motors as a load this should not be a major issue.

So - yes, it will probably work OK. Trying it is easy enough. You don't need a motor controller to start - just connect the motor to the 12V output wide open throttle mode ...). If the 12v supply does not sag appreciably you can probably make it work.  

You need to know about PC power supply starting link, power good, ... bu this is all well covered on the internet.