Graham,
Yeah, 70°C is way too hot for a CPU. A 500W PSU should not be a problem, unless you're running a gaming graphics card and a whole slew of hard drives.

What I recommend is this. Cleaning the CPU fan and heatsink is a good thing, but since the computer has been running that hot, there's a good chance that the thermal compound between the CPU and the heatsink is in bad shape. That stuff is crucial to the life of your CPU.

I HIGHLY recommend using Arctic Silver. http://www.arcticsilver.com/# I use this on all of my computers, and even my video cards, and it will drop the temperature by another 5°C or more. My AMD 3.0+ GHz Quad Core runs at about 43°C.

If you can find a computer shoppe around you that has Arctic Silver, and be careful of the spelling, because there are imitations out there, check back on the site about how to apply it. There is a technique to it. Too much and it won't work, not enough and it won't work. Just the right amount does an amazing job. If you get this, remove the heat sink and use acetone to clean both the heatsink and the top of the CPU, then dry thoroughly with a clean, lint free cloth. Follow the directions on the Arctic Silver website for how to apply the product you choose (I use Ceramique lately, but still have AS 5, too). Reinstall the heatsink, and start the computer. Enter into the BIOS and go to the hardware monitoring page, and watch the temp. It should go up a few degrees and stabilize. Your CPU type, and how much power it draws, will determine the temperature, but at idle, before loading Windows, it shouldn't go up more than three or four degrees. If it goes above five or ten degrees, you'll need to shut down and check to make sure the heatsink/fan assembly is correctly seated.

I would recommend watching this video:

https://www.youtube.com/watch?list=PLB38FA4D49509699D&feature=player_embedded&v=9VSDkB-3st4

One of the other things that I would recommend, and always use myself, is a larger cooler than what comes with the retail boxed CPUs, or worse, factory coolers. Check your mainboard, socket type, and case size, but you can get some really nice coolers now that will run as cool, if not cooler, than liquid cooling.

Gary

I'm blessed watching God do what He does best. I've had a few rough years, and I'm still not back to where I want to be, but I'm on the way and things are looking far better now than what they were!