Virus Help - Slowing Wiping out the network

Dear All,

Please can you help an damsel in distress. Our network seems to have been infected with some sort of Virus. Firstly wiping out the main server (WS2003), this was rebuilt. It now seems that it is spreading through the network and wiping out machines around the site.
I think it may be a boot sector virus, however our virus software (Sophos) does not pick it up (neither does AVG, or CLAMWIN). Once infected, the machine will not boot (not even in safe mode) and we are getting 1 of two error messages “ntoskrnl.exe is missing or corrupt” or “hal.dll is missing or corrupt”
Has anyone else had something similar to this? Or any ideas on how to remove it?
Regards