Need to monitor temperature of room?

I want to be able to attach a tempersture probe? to the PC that will monitor the temparutire of a room and then do certain functioins such as switching fans on and off.

I want to be able to do this as cheaply as possiible

I can programme a comuter if required but not sure what hardware I should be looking for

Any help would be amazing, Thanks

you need a thermal switch