This is fairly easy to do, BUT there is a safety factor you need to consider. If you look to doubledigit.com, you can see my background is embedded electronics, various software and automatic test equipment (ATE.)
In the ATE realm we would add sensors and controls to PCs and then software to do the control function. The only issue can be the cost involved. Some have zero budget and that's a non-starter. Tossing a PC at it is best done not as the central controller but as a Super-GUI frontend to the embedded controller (don't worry, there are many) which allows you to adjust the running parameters and readout the temperatures, switch states and more.
If you want, go to doubledigit and there's some pictures and contact info.
Example embedded PC - http://www.winsystems.com/papers/epc_industrial.pdf
Example temperature measuring for the PC - http://www.windmill.co.uk/thermocouple.html
Example PC104 relay board - http://www.arcomcontrols.com/products/icp/pc104/modules/AIMr8in8.htm
There is a lot of stuff to do this with. What you need are people that have done such or similar before.
I have a general question. Does amybody know of a way of using a pc to control valves and monitor temperatures. I need to monitor the temperature in a tank and be able to open and close hot water valves to keep the temperature constant. I know it's a bit of a bizzare question but any websites, forums, books and info would be greatly appreciated.