Microsoft Knowledge Base Article 815624 contains the following summary:
By default, Windows XP and Windows XP Service Pack 1 (SP1) do not
enable AMD PowerNow! technology for AMD PowerNow!-capable AMD Athlon and AMD
To enable AMD PowerNow!,
you must install the
latest processor driver from
Note The necessary driver support for PowerNow! technology is already configured on most systems sold by original equipment manufacturers (OEMs).