EDIT:
I dont think Speedfan can change my fanspeeds my mobo is ASUS P6T DELUXE V2... i tried tweaking a bit but nothing happened... i also did a bit of research and many do claim that speedfan cant change fan speeds for my mobo as of now...
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You could try contacting the author, but regarding your initial question fans spin at top speed by default. If the system is running hot you need to look at other things. In my experience the main reason systems run hot is dirty heat sinks, so make sure they're clean (blow out the power supply too). Make sure air is flowing through the case properly - case fans need to be pushing/pulling air in the correct direction or else they work against each other. If all of the above isn't enough you could try an additional fan. By the way, are you overclocking?