My laptops "40GB" HDD is split up like this:

C:
Used: 6.9GB
Free: 10.0GB
D:
Used: 0GB
Free: 17.3GB



+ How do I make it all into just one C drive, or do you recommend that I use "C" to install programs on, and "D" to save files on?

Thanks for your help.

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As far as I know you'll have to reinstall Windows to take advantage of all the HD space and create a single partition.
So if you don't want the hassle I'd just use it like you said

Alternatively, PowerQuest Partition Magic or similar partition editor should be able to consolodate your partitions into a single one.