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.
Got an acer aspire t135 with 80g hard drive which was partitioned 40g/40g.
Had virus problems so decided to wipe the whole system and install my windows home I have.
Have now only got a 40g hard drive though
How do I get my other 40g back ?