look closely at the jumpers on your motherboard (if you have or can download the manual, that makes it much easier) and try and find one marked CMOS clear, or some variant of it. If it's a 2-pin jumper with a cap on it, pull it, turn on power, turn it off, and reinstall the jumper cap. That should do it. (if it *doesn't* have a cap, install one to discharge the cmos. I haven't seen too many of those, though.)
(actually, I misspoke, you shouldn't be poking around with the power on anyway, especially on ATX systems- to avoid damage, pull the power cord before changing memory or any cards, since soft-off often will *not* kill power to those areas! New boards are finally coming with little LED's to remind you of this, but many haven't got any indicators)
If it's a 3-pin header with a cap on 2 pins, power down, move the jumper to cover the center and previously open pin, then move it back. If this doesn't do it, do the same, but briefly power on the system before moving it back. this should clear 99%+ of the boards out there.
One final trick I got from Gigabyte when my GA-7VAXP's CMOS became read-only is to power down, then remove the battery for 48 hours (yep, 2 days) before reinstalling the battery and trying the system again.
Also, any of these that work *will* dump time, hard drive, and any other BIOS settings for the whole CMOS, and reset everything back to 'factory default'. Good luck!