I can not completely format a hard drive,which was originally a disc 'C' and I put it as a slave drive into another computer.It's still containing some user data and something hidden and I can not access erase it. I have Windows XP
You may have several partitions on it. Go into Drive Manager, you will see this second drive listed, remove any partition on it. Then create a new on in the same place, make sure it gets a drive letter assigned. Then format it. All of this can be done from the same screen.
the option "format " is not highlighed,can't do anything
Are you going though Disk Manager or though My Computer? Delete the partitions first, then try to format. You can also try gparted live but be very careful when using that as you can easily wipe your main disk with it. You may want to disconnect your main drive if you use it as it's a bootable CD anyway. http://www.pcworld.com/article/226231/GParted_Live.html