They should not get erased. All of the backup programs I'm familiar with simply create a separate backup file. Just make sure you have enough free space on your external HD. (Before a backup begins it tells you approximately how large the file will be.)
Have you decided which backup program you're going to use? The most popular seem to be the Windows 7 backup / imaging utilty, Macrium free, Paragon free, or EaseUS ToDo free. If you're given the opportunity to create a separate companion System Repair Disk on a single CD, please do so. It could help you boot to system recovery options if you don't have a Windows installation disc, can't find your Windows installation disc, or can't access the recovery options provided by your computer manufacturer.