All Ubuntu guests fail to start

Hi folks,

Host - Debian 5.04
KVM

I have Debian/Windows/Ubuntu guests running on the box. For unknown reason recently all
Ubuntu guests can't start. Other guests are working. Always hanging on

Code:

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kernel_thread_helper+0x7/0x10

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Just tried installing a new Ubuntu 10.04 guest without success. The guest started to
boot but hanging on the screen finally.

Please help. TIA

B.R
satimis