On the mainboard, if it doesn't work, or if you want to know ahead of time if it should work, check what the mainboard model is for us, and post it back.
eg: "Gigabyte GA-8PENXP Rev 1.2" or something like that.
Even Dell, HP / Compaq, etc mainboards should all have part numbers printed on the board in white, near the video card slot usualy.
If it doesn't work a BIOS update should be all you need.
You'll be wanting to use NTFS (vs FAT32) for a disk that size though for various reasons I won't go into unless you PM or é-mail me.
If a 80 GB HDD works, then chances are a up to 120 GB will also work fine, and maybe even larger.... this is assuming the interface (connector plug) is for ATA100/ATA133 and not Serial ATA which her board is unlikely to support.