On Windows XP it's in the registry: 
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\HARDWARE\DESCRIPTION\System\CentralProcessor\0]
"Component Information"=hex:00,00,00,00,00,00,00,00,00,00,00,00,01,00,
+00,00
"Identifier"="x86 Family 6 Model 15 Stepping 6"
"Configuration Data"=hex(9):ff,ff,ff,ff,ff,ff,ff,ff,00,00,00,00,00,00,
+00,00
"ProcessorNameString"="Intel(R) Core(TM)2 CPU          6600  @ 2.40GHz
+"
"VendorIdentifier"="GenuineIntel"
"FeatureSet"=dword:a0073fff
"~MHz"=dword:0000095a
"Update Signature"=hex:00,00,00,00,c6,00,00,00
"Update Status"=dword:00000000
"Previous Update Signature"=hex:00,00,00,00,44,00,00,00
"Platform ID"=dword:00000001

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On Debian/GNU (Lenny) Linux it's in /proc/cpuinfo:

processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 15
model           : 47
model name      : AMD Athlon(tm) 64 Processor 3000+
stepping        : 2
cpu MHz         : 1808.830
cache size      : 512 KB
fpu             : yes
fpu_exception   : yes
cpuid level     : 1
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
+ mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx mmxext fxsr_
+opt lm 3dnowext 3dnow up pni lahf_lm
bogomips        : 3619.93
TLB size        : 1024 4K pages
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management: ts fid vid ttp tm stc

[download]

In Windows 98SE it might be in the registry:)

REGEDIT4

[HKEY_LOCAL_MACHINE\Hardware\Description\System\CentralProcessor\0]
"VendorIdentifier"="GenuineIntel"
"Identifier"="x86 Family 6 Model 8 Stepping 10"
"Update Status"=dword:00000002

[download]

It's a Celeron 900MHz, IIRC.

update: Added 98SE
update^2: Corrected 98SE entry

In reply to Re: Current processor frequency? by FunkyMonk
in thread Current processor frequency? by BrowserUk

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