For the highest and most repeatable benchmark scores on AMD processors running Windows Vista or Windows Server 2008, AMD strongly recommends using the "High performance" power management power plan. This setting ensures the processor cores are always running at their maximum rated clock speed. The power plan selection window can be accessed via Control Panel -> System and Maintenance -> Power Options (default) or Control Panel -> Power Options (Classic View).

There are alternative methods like powercfg that offer a scriptable way to manage power plans. The exact parameters can be investigated by opening a command box (CMD) and entering "powercfg /?" to get a list of options.

The GUIDs for the various scheme can be discovered by entering "powercfg -aliases".

C:\Windows\system32>powercfg -aliases

a1841308-3541-4fab-bc81-f71556f20b4a SCHEME_MAX
8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c SCHEME_MIN ?High performance i.e. min powermgt
381b4222-f694-41f0-9685-ff5bb260df2e SCHEME_BALANCED
fea3413e-7e05-4911-9a71-700331f1c294 SUB_NONE
238c9fa8-0aad-41ed-83f4-97be242c8f20 SUB_SLEEP
29f6c1db-86da-48c5-9fdb-f2b67b1f44da STANDBYIDLE
9d7815a6-7ee4-497e-8888-515a05f02364 HIBERNATEIDLE
94ac6d29-73ce-41a6-809f-6363ba21b47e HYBRIDSLEEP
b7a27025-e569-46c2-a504-2b96cad225a1 CRITICALSLEEP
7516b95f-f776-4464-8c53-06167f40cc99 SUB_VIDEO
3c0bc021-c8a8-4e07-a973-6b14cbcb2b7e VIDEOIDLE
90959d22-d6a1-49b9-af93-bce885ad335b VIDEOADAPT
0012ee47-9041-4b5d-9b77-535fba8b1442 SUB_DISK
6738e2c4-e8a5-4a42-b16a-e040e769756e DISKIDLE
4f971e89-eebd-4455-a8de-9e59040e7347 SUB_BUTTONS
7648efa3-dd9c-4e3e-b566-50f929386280 PBUTTONACTION
96996bc0-ad50-47ec-923b-6f41874dd9eb SBUTTONACTION
5ca83367-6e45-459f-a27b-476b1d01c936 LIDACTION
a7066653-8d6c-40a8-910e-a1f54b84c7e5 UIBUTTON_ACTION
e73a048d-bf27-4f12-9731-8b2076e8891f SUB_BATTERY
637ea02f-bbcb-4015-8e2c-a1c7b9c0b546 BATACTIONCRIT
9a66d8d7-4ff7-4ef9-b5a2-5a326ca2a469 BATLEVELCRIT
d8742dcb-3e6a-4b3c-b3fe-374623cdcf06 BATACTIONLOW
8183ba9a-e910-48da-8769-14ae6dc1170a BATLEVELLOW
54533251-82be-4824-96c1-47b60b740d00 SUB_PROCESSOR
bc5038f7-23e0-4960-96da-33abaf5935ec PROCTHROTTLEMAX
893dee8e-2bef-41e0-89c6-b55d0929964c PROCTHROTTLEMIN
68f262a7-f621-4069-b9a5-4874169be23c PROCIDLE
bbdc3814-18e9-4463-8a55-d197327c45c0 PROCPERF
0e796bdb-100d-47d6-a2d5-f7d2daa51f51 CONSOLELOCK
501a4d13-42af-4429-9fd1-a8218c268e20 SUB_PCIEXPRESS
ee12f906-d277-404b-b6da-e5fa1a576df5 ASPM


C:\Windows\system32>

A power scheme can be queried and set by entering "powercfg -setactive <Scheme GUID>".

C:\Windows\system32>powercfg -getactivescheme
Power Scheme GUID: 381b4222-f694-41f0-9685-ff5bb260df2e (Balanced)

C:\Windows\system32>powercfg -setactive 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c

C:\Windows\system32>powercfg -getactivescheme
Power Scheme GUID: 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c (High performance)

Other references:
» Processor Power Management in Windows Vista and Windows Server 2008

This Microsoft paper describes commands to disable C-state transitions starting on page 31, as well as an in-depth look into power management, and methods to measure the impact of power management with Perfmon.