I would go with RealTemp or CoreTemp than. The one advantage to RealTemp is you can calibrate your sensors to get a more accurate reading. I would just stay away from SpeedFan, as in my experience it isn't as accurate.
CoreTemp wants to "make changes to my computer". Why?
What OS are you running?
It doesn't make changes to your system but that is a standard warning coming from Vista or Win7 with the UserControl set at default.
Windows 7. It's a standard warning when a program is making a change.
There is nothing to be worried about when installing CoreTemp. It isn't changing anything, just install the program, which technically is a change to your system
Which PC do you have?
Dell Studio Desktop 540.
Core temp should work fine. It will ask you if it can make changes becuase it access hardware. I use Windows 7 64bit with a C2Q. does it for me. I've not tried Real Temp on this machine so I can't say.