I bought my Dell Studio XPS a couple of months ago, and whenever my CPU goes above 18% ~ it starts to overheat, and the fan runs very loudly. My Task Manager at 5% CPU: CPUID HW Monitor test done at 5% CPU: Now if I play some type of computer game/process and the CPU overlocks/overheats; My Task Manager at 18% CPU: CPUID HW Monitor test done at 18% CPU: I am also running Windows 7 on a 64 Bit PC.
I suggest you do some measurements inside the case and find out the largest aftermarker CPU cooler you can install. The Coolermaster TX3 and Hyper 212+ are very good coolers for the cost. What sort of case ventilation does your Dell PC have? I'm assuming 1x 120MM exhaust fan? Is there much dust buildup inside your computer?