cpu fan error, chasis introded, fatal error...system halted.

reload the BIOS as it may not have setup properly.

< Edit >

Missread the Post Sorry

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there is a setting in BIOS that is set to alert you when the chassis has been opened. You can disable intrusion detection in BIOS, or clear the message.

I suppose it is possible that intrusion detection may also be set to disable the system.

See this bit:
"ASUS CrashFree BIOS2
The CrashFree BIOS2 feature now includes the BIOS auto-recovery function in a support CD. Users can reboot their system through the support CD when a bootable disk is not available, and go through the simple BIOS auto-recovery process. ASUS motherboards now enable users to enjoy this protection feature without the need to pay for an optional ROM."

http://www.asus.com/products.aspx?modelmenu=1&model=765&l1=3&l2=11&l3=229

edit: see also
http://support.asus.com/download/download.aspx?SLanguage=en-us&model=P5V800-MX

for any related downloads, including a handy manual.

Fan:
This error message represents system detected either the onboard CPU fan connector is not connected, or if CPU fan speed detected during POST stage has gone below the lower limit.

The possible causes of system showing such error message includes:
1. Onboard CPU fan connector has not been connected to 3pin or 4pin CPU fan.
2. CPU fan speed at POST stage has gone below 600RPM for Intel motherboards, and 800RPM for AMD motherboards.

If your CPU fan has low speed, or CPU fan has only 2 pin, or you are not using CPU fan connector, you can bypass this error message by entering BIOS setup screen, and set CPU Fan Speed」option to Disabled」to bypass this error message at POST.

Is it at all possible that various connectors or jumpers have been changed accidentally? Both the fan and intrusion detection are set with connectors on the board.