I phrased the title poorly (sorry).

I want to add another hard drive to expand the storage space from 80 gigs to 120 gigs (Adding a 40 gig). I know there is something i have to do to set the second drive as a slave but I'm not sure what. When I plug in both drives at the same time the computer doesn't see either, however when i use either the 80 gig or the 40 gig alone it boots fine.

I run vista ultimate 32 but that shouldn't be of consequence because the computer doesn't see the hard drives.

I'm not certain what kind of drives they are (SATA / IDE) or how to tell, but I think they are IDE.

any help would be appreciated.

do the drives connect to the pc via the same cable?

there is a good chance both are set to "master" mode on the drive itself, it will look something like this maybe

MSC
ALS
:::
with a small platic clip between 2 of the pins, move the pin on both drives to the oposite end position seting both drives to cable select and you should be laughing. Otherwise set our cd drive to slave and attach the 2nd hdd to the same cable as the cd drive.

Just look at the diagrams on the HDDs, it should tell you where to put the jumper (the removable plastic thing in the back) to set it as either master or slave. I always have problems with cable select so set one as master and one as slave and plug them into the same cable to make sure they don't conflict with your cd drive.