Dell hidden partition?

Your password may not be long enough.

I would keep that partition and transfer it over. It may be a hidden partition with capacity to restore your system to factory defaults. It probably has an operating system and drivers. To transfer it clone the old disk to a new one.
You can likely get a FREE disk-cloning utility from the website of the manufacturer of your new drive. From Seagate you can download and install on your current C: drive a package called Seagate Disk Wizard, which is a customized version of Acronis True Image. From WD you can download their Acronis True Image WD Edition. Other disk makers have similar tools available. In general, these tools are set up so they will make a clone only TO a drive by that same maker. They don't care whose older drive you are replacing, but they are happy to help you move to one of their drives.
Hook up both drives, clone to the new drive. Turn the computer off. Remove the old drive. Make sure you are booting from the new drive. You can use the old drive for backups or extra storage. You can put it in the computer or buy a hard disk enclosure and use it as an external hard drive.

Okay, I'm cloning the current 80GB drive to the new 640GB drive. I would have liked to just clone the EISA FAT16 partition, but I couldn't figure out how to do that in Acronis TI WD. So I guess I'll just clone the entire drive and then delete the cloned C: partition, recreate a new C: partition on the entire remaining portion of the drive (remaining after the ~400 MB FAT16 partition) and install Win7 on that new partition.