1. Get to the drive management console (in XP? You didn't say) and see if you can activate the second half.

2. If you had split the drive after the machine came from the factory, then you may have used the restore CDs. These won't respect your custom partitioning so I suspect what's on the second half may vanish as you run the drive management console to format that second half.

You didn't reveal the OS or exactly what you did so I'm left to wonder a bit.