How do you undo a partition on a hard drive?

I partitioned my hard drive without really knowing what i was doing so now there's a c and a d drive. How do I undo this so it's just one large c drive?

I think you'll have to erase both partitions and then create one large partition, of course this will involve erasing your hard drive so you'll need to reinstall everything.

I think PartitionMagic can combine the partitons...