Before you do anything, make sure you have backed up your data. I'd also recommend using a program like Acronis True Image to make images of your partitions and a copy of your boot record. Then, if anything goes awry you can restore what you had before you fiddled with the computer. Do you have operating system disks? If so, you could also use those to reinstall your operating system if anything gets completely messed up (make sure you've got copies of all of your data as well).
You can resize your partitions with a program called Gparted which is available for download here Gparted
What operating system do you have? I've used Gparted in xp without any issues, but the bootmanager in Windows 7 got completely messed up by it. I believe that Vista has a similar bootmanager to Windows 7 so it might cause booting issues if you've got vista.
Do you have your data on C:? Is your backup partition definitely only reserved for backups, or would you be happy to move your data there? You can find instructions online as to how to move your data folders to another partition (how to do it depends on your operating system). This is a much easier option if you are happy to use the existing partition for your data.