k4gicg is right - the disk the unit boots from WILL be called C: by default. Now, your problem appears to be that you also have at least one other hard drive that was called C: and now does not show up. You may be able to fix that by giving it a new letter name.
Click on Start in lower left, and in that menu RIGHT-click on My Computer. Choose Manage from the mini-menu and in the new window on the left, expand "Storage" if necessary so you can click on "Disk Management". This will show you two panes on the right. Look in the LOWER RIGHT pane. It scrolls to show you all the hardware drive devices it has. You will see your newer HDD there, your older HDD, your optical drives, and any other HDD's around. Each is represented by a large horizontal block. Within that block is a small label on the left end with stuff like "Disk_2", a size, and a couple other bits if info. To its right are larger sub-blocks, each representing one Partition on this HDD unit. A HDD may have only one Partition, or it could have more. Each Partition sub-block will have a letter name like C:, a size, a File System, and a bit more info. So your new HDD unit with Win 7 mounted on it will show up here with a major (maybe its only) Partition labeled C:, etc.
Now, look for that older HDD you cannot use. It should be there, and its major Partition probably does NOT have a letter name. It used to be C:, but that name got stolen! So, you can RIGHT-click on this sub-block and choose the option to Change its name, giving it any letter not in use. When done, exit out of Disk Management and reboot so the Registry is updated. Your old drive should now show up in My Computer with a new letter name and all its old files available.
You may have to go around to all your icons that link to files and applications on that old drive and edit them so they point to the new drive's letter name, not to C:\(something...).