>>1
Look for something called IDE ATA/ATAPI controllers or similar. In here it'll list the SATA and ATA controllers on your motherboard. If there are only ATA controllers or SATA controllers then you are done.
If not, right click a controller, goto properties and look at the "primary channel" and "secondary channel" tabs. On each tab it will show you the master or slave drives on that channel, if any. The devices you will find here will be things your Hard disk drives, CD drives, and DVD drives. Now you can either search until you find the HDD you want to replace (and thus figure out what interface it is) or you can search until you find a free spot, and buy a drive with whatever that interface is. SATA is the newer and superior technology so definitely choose that if you can.