I had the same problem last week, I was geting my harddrive ready to be placed in to my new build. So I formatted my hard drive and then needed to change my boot settings in order to load xp from disc.
I removed the battery but with no luck, there must havce been some residual power being stored somewhere........don't know where though.
After reading the motherboard manual for my new comp I saw that there was a relavent jumper.
So I just removed the jumper and replaced it.
That did the trick, it was the closest jumper to the cmos battery.
Hope that helps, if not then I hope it was a slighty interesting to at least someone out there.