When you take an image of a drive, it needs to be a sector copy of the entire physical drive. This means that the image is an identical copy of all data on the disk.
It is possible to do this with programs such as GHOST - but usually the sector copy is not the default option (programs like this tend to want to save file size by only imaging the active file space).
There are a bunch of forensic image tools available, the most common being EnCase (but also very expensive).
There are some free ones available, such as EzyImage from
GetData Download Center: Data Recovery, File Repair and Windows Software. This one is very simple and does not provide the option of compression - so you image file will turn out to be the same size as the actual hard drive.