need computer help please?

I assume you're using Windows XP. Right-click My Computer and select Manage. Wait for the Computer Management window to come up, then select Storage. Then double-click Disk Management. You'll see your hard drives on the right side of the window. Select your newly installed hard drive or right-click on it and you'll be given choices to format it. Once you've assigned a drive letter to it and formatted it, you will then be able to use that drive.

Hi

Go to computer management under control panel and from there assign a drive letter to the drive if still it does not works try formatting it

Is your hard drive shown in your explorer??
If not then try changing the drive letter through the windows control panel. If this is not working then you have to format it. You can also try using some softwares such as partition magic to see the partition and drive details.

Make sure that parameters such as drive letter, boot directory are not same for both the drives.
You can also change the bios settings for the drives if it is not working in windows.