you'd need software like acronis true image or norton ghost to make an image .
OC wont affect your hard drive and data stored on it btw , and if it did then any image on that drive in another partition would be lost as well
Actually you don't need any backup software to create a system image with Windows 7, I'm currently using a leaked copy of Windows 7, final build. There is a Backup and restore utility with which you can create a system image. Overclocking doesn't affect data, but it might impact system stability, which may cause the system to crash or BSOD.