Cpu is overheating and I don't know why

PC Specs:

Asus A7V MB
Duron 1.2 Ghz
512MB PC133 ram
80gb Maxtor
100gb Maxtor
Geforce 4 MX
Windows Xp Pro
Case Edge Case with 300W PS

When I get into windows within 30-60 minutes the cpu is at 55 degrees Celcius and continues to rise to about 70 degrees celcius. As far as I'm aware the norm temperature is around 45 degrees celcius. When I replace the heat sink fan with a different one the temperature tops at about 65 degrees celcius. Still really high. The case is open with a lot of airflow. If I place a really large fan on the motherboard area the CPU ranges from 55-60 degrees celcius.

What could be the cause of this high temperature? What can I do so fix this problem. Any suggestions or help would be greatly appreciated.

i would say reapply the thermal grease. ive got a 1.2 duron and it will occasionally reach 50C, but only because im running it at 1355 on air...

I've reapplied the thermal paste it doesn't help any.

Is the CPU fan blowing air down on the processor or up away from the processor? Check that (Down Please!).

What type of thermal grease are you using? You want to user Arctic Silver 3. Lapping the heatsink would also help before using the AS3. And on the subject, what kind of Heatsink do you have? What kind of fan do you have on the heatsink? And since I didnt see it mentioned, are you overclocking? There are more questions, but I'll save them for later.

-YB