Did you try to install this one:
http://www.amd.com/us-en/asset...t_ty...amdcpusetup.exeCODE
AMD Opteron™ Processor with AMD PowerNow!™ Technology Driver for Windows XP and Windows Server 2003 Version (x86 and x64 exe) 1.3.2.16 - Allows the system to automatically adjust the CPU speed, voltage and power combination that match the instantaneous user performance need. Download this Setup Installation program (EXE) to automatically update all the files necessary for installation. This package is recommended for users whom desire a graphical user interface for installation. This .EXE driver is a user friendly localized software installation of the driver designed for end-users. This driver supports Single- and Dual-Core AMD Opteron™ processors on Windows XP SP2, Windows 2003 SP1 x84 and x64 Editions.
(copy&paste from AMD... yes, they wrote "x84" - whatever...)
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Regards,
  Ghostrider
·CPUs - 2*Opteron 246
·Mainboard - Tyan Thunder K8W S2885ANRF
·RAM - 4*Corsair TwinX1024RE-3200LL (4GB, 8*512MB, DDR400, 2-3-2)
·Graphics - HIS IceQ-II X800Pro
·HDD - Western Digital Raptor WD740GD, 74GB, 10k
·RAID - 3Ware 7850, 64bit PCI, 8chn hardware RAID
·RAID-HDDs - 5*WD800JB
·Optical - 1*SlimSize CD-ROM; 1*NEC7441 DVD-RW
·Sound - Creative SoundBlaster X-Fi Elite Pro