Is there any Windows 7 gadget that can show CPU and hard disk temperature without installing any extra software such as Speed Fan?
You can't access them without installing extra software. SpeedFan, Hardware Monitor by CPUZ people and the utility the motherboard maker provides are the best.e
Here is some more info about CoreTemp and RealTemp.
The CoreTemp has a sidebar gadget as well see here. In order for gadget to work first you need to install the CoreTemp and than add the addon for the gadget.
There is also a hardware diagnostics and monitor tool called Everest. (to enable the gadget: open everest and let it run in the background. Then go to gadgets and it will show up there ... that`s all... to enable the gadget go to everest preferences and choose what information you want it to show).
Finally, many people recommend using SpeedFan (see Monitor Your Computer Fan Speed With SpeedFan). There is a way to show its data on the sidebar but I don't recall how to do it. Try googling it.
You might be also interested in the following article from about.com. While it doesn't list the temperature gadgets specifically there are several others you may like.
Top 16 Windows 7 Gadgets for System Monitoring
Yeah, speed fan is a very light program, but it does require installation. You don't want a website to check your system health. in general it's not a good idea. There is also realtemp:
Core Temp is a small utility that monitors the temperature for Intel and AMD CPUs. It doesn't require installation. I couldn't find a similar tool for monitoring HDD temperature.
What's the problem with installing Speed Fan?
Speed Fan will not read my hard drive in Windows 7 on a Asus 6850
In that case I suggest using another tool.