It's automatically detected provided your motherboard supports that size. For example, most older motherboards don't support drives over 80gigs - and some do only wirth a firmware upgrade. Also, make sure you set the drive's jumper settings according to the manual.
What happened was, I got a new (to me) tower. I was using my old one as an exchange. I had slightly upgraded my old tower and it had a bigger hard drive than the new one. The guy said he would switch them out free of charge. He was supposed to have put my 20 gig drive into the new tower. Now I keep getting a message that my drive is full. When I check the properties, it shows only having 1.98 gigs of total space. I didn't know if he forgot to change the settings from the 2 gig to my 20 gig or if I just got screwed!
All you have to do is go into your "my computer" and find the hard drive. Right click and select properties. There it will tell you how big the HD is and how much has been used. You will know if you got screwed.