By 120+30GB, I assume you mean your old hdd is 30GB, and the new one 120GB?
If you are having 2 hdds on the same channel (using the same cable), you should set the jumpers of one hdd to master, and the other to slave.
The one which you intend to have Windows installed on it should be master. [So if you want to install a new copy of Windows to the new hdd, set the new hdd as master, and the old one to slave.]
According to the online manual, your motherboard's BIOS setup has different "Modes" for the hdds. You will need to select at least LBA (or Large - but I don't know what the difference between LBA and Large is, for your motherboard).
When you have properly connected the hdds and set the jumpers properly (master/slave), turn on computer. Press Delete to enter BIOS setup.
Be careful in BIOS setup. Don't change anything you're not sure about. If you think you may have changed something accidentally, just exit BIOS without saving changes. Then try again.
Once you're in BIOS setup, on the first screen, to the right, make sure its says IDE HDD AUTO DETECTION.
If so, choose Standard CMOS Setup. Then select the disk in question (primary master or primary slave or whichever one it is). Select the "Mode" option. Then you should try LBA first.
Then Save BIOS settings changes and exit BIOS Setup. Computer will restart and this time see if the hdd capacity is properly detected.
If not, go back into BIOS setup and try the "Large" Mode.