CPU high usage

[trolo_vk]

Hi,

ZIMBRA running on my server:
HW:
CPU: model name : Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz
RAM: 8GB
SWAP: 512MB
software RAID 2x 600GB
SW:
XEN
uname -a
Linux IN-XEN1 2.6.26-2-xen-amd64 #1 SMP Wed Mar 10 00:29:48 UTC 2010 x86_64 GNU/Linux

I created XEN machine for zimbra.
OS: debian 5 (lenny) 64-bit
CPU: all 4 core of CPU
RAM: 5GB
SWAP: 512MB

here is usage my CPU...
DAY
cpu-day.png
MONTH
cpu-month.png

i don't now why CPU usage is high.
no users login in zimbra and cpu USAGE is high, too
help me? thank you