My PC's clock is off badly every time I boot if my PC has been unpluged for a while. I unplug my PC when I'm at work and I've noticed lately that the clock is off by several hours when I boot. The funny thing is the hours is what gets off, the minutes stays accurate and the date is sometimes wrong.
I never have had a problem with mine. I even have an old Pentium 233MMX System that keeps very accurate time on teh origonal battery. Any ol' CR2032( i think thats the type) should do good for ya.