How Can I Find Out The Condition Of My Cmos Battery And How Do I Know It S Weak

Num Lock gets off while entering the password for user and also time gets changed frequently.

Thus I suspect the CMOS battery is weak. If not, how do I check it and why does Num Lock turn off at the time of entering password and why does only the time change, not the date?

I am dual booting between XP and Ubuntu. I have a virus free computer as I am using Avast Pro in XP. Linux does not have any type of virus.