Walman chap, I feel sorry for you, normally HDD's are a piece of cake to install.
Even if you have to bodge where they're sitting, normally just a case of:
Remove case panels, set the jumpers to suit (normally slave if it's an extra IDE, if SATA no jumpers at all) fit and screw in HDD, attach cables + minor moving about, put case back together and bingo.
S' a bit off you had to fettle with the BIOS though to get it to find the drive, I've never ever had that problem.