Unable to use Windows 7 backup

Hi,
I bought a 2nd gen Dell 14R laptop a month ago. I haven't used it yet, because I want to back it up before anything bad happens to it. So I am trying to back it up onto an iOmega 500GB external hard drive, using Windows backup.
However, when the information is backing up, it only gets to about 60%, before it stops and says that an error has caused the back up to fail.
The error code is 0x8078002A. I also tried to backup my sister’s new 2nd gen Dell 15R laptop using the same device and I received the same error code.
I do not know what else to do to make the back up successfully. I have been working on it for a month.
