Brand new macbook - need to change two HD partitions to single patition?

My sister has just bought a new macbook running snowleopard and I'm trying to configure it for her. I work in web so I'm IT literate but I'm not too familiar with macs.

It has a 250Gb HD split into two 125Gb partitions - the 'macintosh HD' (with all the apps, system files and such) and a 'data disc' (which is currently empty). I'm not sure why it has two partitions that's just the way it came.

I want it configured as a single 250Gb partition. I've gone into disk utility and deleted the 'Data Disc' partition. So far so good. Now I want to 'extend' the single macintosh HD partition so it uses the full 250Gb of the physical disc. But there is no handle on the Mac HD partition so I can't extend it and all the options are all greyed out so I can't change anything. It's like it's forcing me to have two partitions.

How can I make it a single partition?

Thanks in advance.

Welcome to Mac Forums
This sounds bizarre! I've never heard of this, but anyway....
You need to boot from the install disc (boot holding C as soon as you hear the chime until you see the spinning pin wheel).
Select the language, then go to utilities>disk utility in the menu bar and drag out the partition to fill out the space.
Once you are done, exit utilies and restart.

Thanks 6string. I'll give that a go.