Use the BELARC advisor and see what it reports.
Recently I bought a PC. The CPU was described as "P42.8 Northwood socket-478." However, in the bootup sequence it is described as, "Intel(R) Pentium(R) 4 CPU 1.40 GHz." System Information has two identical lines with the following text: [Item] "Processor" [value] "x86 Family 15 Model 2 Stepping 9 GenuineIntel ~1397 Mhz".
Is it clear from this what the Mhz rating of my CPU is, or is there a way to find out without ripping the fan off?