what caused bios reset?

hi, pc has had no problems for ages, i just removed a load of registry entries, mainly from zone alarm and true vector, i reset winsock and rebooted, went for a shower and came back to a boot failure message. bios had reverted to default.
it was showing correct time and date but had put everything back where it should be and was trying to boot from the external hard drive. everything seems ok now, i have reflashed bios with an updated version, but i thought bios was stored on a chip? so how did i break it doing some standard software cleaning?
as i said it seems fine now it's just baffled me a little.
thanks adam

  Quiller. 01:50 25 Feb 08

when you reboot and the bios comes up with a cmos checksum error. This can because by hardware or by altering windows setup files for the hardware or software.It will reset the bios to defaults.

You've obviously altered windows enough via the files to reset the bios.

thanks for that, i shall leave messing for a day or two, or try to anyway.
thanks again.
adam

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