Internal hard drive not detected on boot

Hello,
I got this cheap second hand computer (dell gx260) from a work place but it had no os.
1.80ghz, 512ram etc ... I manged to install the os and all the drivers and i was working fine for a while. I decided to add another 80gb hard drive that was used with my old computer as the current one only had 20gb but it could not detect either hard drive..... I disconnected the old one and just tried to run the computer on the 20gb again but it still coundnt be detected .... pls help!!! And are there any advisable upgrades as i just want to use this computer on my tv to watch downloaded movies. thks