How to increase CPU core speed of a guest to maximum of Citrix Xenserver Host

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http://serverfault.com – There is a Citrix XenServer 5.5 (HP ML370 G5, Xeon 2GHz) on this Server there is a Windows Guest (Windows 2008 R2 x64) under Properties I set 4 CPU Cores with max prio. But if I check with CPU-Z the performance of the CPU from Host there is set the right CPU (XEON 2GHz) but with only 600MHz what is wrong with this Guest. The /proc/cpuinfo from Citrix XenServer: processor : 0 vendor_id : GenuineIntel cpu family : 6 model : 15 model name : Intel(R) Xeon(R) CPU E5335 @ 2.00GHz stepping : 7 cpu MHz : 2000.016 cache size : 4096 KB fdiv_bug : no hlt_bug : no (HowTos)