Ive installed a second HD as a slave but computer hasnt detected it

Keith on the disk that you have set up as a slave you have to change the jumpers on it, so the computer will recognise which one is primary and which one is slave. You will find a little series of pins(normally)between the power supply and the ide cable. Look on the disk for the settings to change it to slave then turn on your computer and autodetect the hard disks in your BIOS (settings).

I don't know your experience level, so if I'm preaching to the choir please forgive me.
For some reason, everyone assumes everyone knows how to install a hard drive. When you install a second harddrive there are several steps. 1)make sure the jumpers are set to slave on the 2nd hard drive. Also, on drive 0 (first drive) some manufacturers have no jumpers is single drive, if dual drives you must put on a jumper. Make sure the jumpers are correct on both drives. Then go into your bios and set up each of the drives (new systems you can just set all 4 drives to Auto (except Dell) and just let the system autodetect. Once you are sure your system can recognize the drives properly, THEN the slave drive must be partitioned. You do that with the program FDISK. You can run this from a boot disk or from windows. If you want to use the entire slave as 1 drive then create 1 partition of maximum size. (This is where it can get fun because some systems won't recognize a partition bigger than 2gb, some 8.4gb, some anysize. If your system won't recognize the size of your drive you will have to flash your BIOS or use a third party disk manager, such as EZ DRIVE. After getting the partitioning done, you must then format d: and format the disk to the operating system you are using. Once this is done, windows or any other operating system should recognize the drive and be able to write and read to it as normal. Hope this helps and didn't insult you with the really basics.