Using Time Machine with a already used NTFS disk I have just updated to Mountain Lion 10.8.2; I have done a clean install.
Now after I have installed all my apps and configure them I would like to create a Time Machine backup; so I could have the image of a fresh clean mac :D

The problem here is that I have only one external HDD (1Tb, NTFS formatted and MacFuse installed, so there's no problem in read/write the volume) and half the disk is used with my docs, pics, music, etc.

Is it possible to create a Time Machine backup without erasing the previous contents ?  

Will Time Machine save the backup just as easy as copying a disk image to the HDD without erasing my files? Or will it reformat the disk?

Greetings and thanks for the help :D
Time Machine requires an HFS+ partition on the drive, it can't use an NTFS formatted one, even with MacFUSE installed. The way to do this would be to shrink the NTFS partition, then make a new HFS+ formatted partition to use with Time Machine (you can do this with Disk Utility). Unfortunately OS X can't resize NTFS partitions, you'll need to use a Windows machine to do it, then use OS X to create a new HFS+ partition.