Set Square wrote:
OK, the title doesn't tell the whole story, because I am using a CH stat for
a non-standard purpose.
snip
I've been having problems keeping the inside of my computer, and the
processor, cool - even after adding extra DC fans. So I have solved the
problem by fitting one of Mr Maplin's 120mm mains-driven fans in the side of
the (tower) casing - which blows quite a lot of ambient air past the
electronics, exhausting through the rear.

http://www.ebuyer.com/
Lots of stuff for doing this sort of thing, very cheaply.
Lots of options, from thermostatically controlled fans to little control
panels that can be set to run fans at certain temperatures from multiple
temperature probes, and have lots of blinking lights on.