Strange Happenings

I took the SATA hard drives out and put an old IDE hard drive in. The BIOS recognized it and all I had to do was to reinstall the Operating System.

How do I go back to the Serial ATA drives which are much larger. The old Hard Drive is 40 GB and the the Serial hard drives are 120 and 160.

Any help would be appreciated. Also thanks for all your help - I have tried everything suggested and it has really been helpful.