!!!HELP HDD INSTALLATION!!!

Hi guys, I've bought a 120 Gb hard disk (maxtor ata 133): I wanna use
it as slave disc replacing an old 40 Gb disc. I've linked t to the
power source and to the data cable putting it in the centre of cable;
I've followed instructions and I havn't put any jumper to set disc as
slave. I keep at the end of the data cable the other hard disc (maxtor
20 Gb) that is set as master. Starting Windows Xp the new disc doesn't
appear, though It happens in "Devices Managment" near the old disc.
I tried to use "find new hardware" from Control Panel, to change the
position of the jumper but there aren't news. Can someone help me?