I use Lavalys Everest (Lavalys - Comprehensive IT Security and Management), and it works like a charm! It even has its own gadget for the Vista Sidebar, where you can display temperatures, voltages, fan speeds etc.
@sEan try and download the Temperature Monitor tool frm your
Motherboard manufacturer's website.
If you have a Core 2 Duo CPU then use a program named "Coretemp".Its very good for temp monitoring.
Speedfan and Everest will also work as the others have said.