Windows 7 32-bit: format/combine two physical hard disks as a single partition
Take a look at this guide:
http://www.howtogeek.com/howto/36504/how-to-create-a-software-raid-array-in-windows-7/
EDIT: To clarify: In this guide a spanned volume is made, which has no fault tolerance. That means that when 1 disk goes down it takes the whole setup with it.
You can go with something like mirrored volumes. This is a more secure option, but the drawback is that the storage space isn't as efficient as it could be with no fault tolerance, it is up to you to decide this.
Take a look at this document for more information.