Can I use eraser for erasing complete hard disk(including boot partition ) by using a live CD or something? . Or use as a substitute for DBAN? Also can I use eraser to erase a complete secondary drive or a non system partition?
The answer is mostly 'no'. There is no bootable Version of Eraser, and never has been; that's why Eraser 5 included DBAN. And, as you are clearly aware, you cannot use Eraser to clear the system drive from which it is running. Development versions include a (not fully tested) capability to erase a non-system partition. With a system drive, you could remove the drive temporarily, use a caddy to connect it to another machine, and erase it from there. But, as your target machine is a laptop, that may not be feasible unless the drive is easily removable.
Personally, in your situation, I would use whatever system the laptop comes with to restore it to factory condition, and then use Eraser to wipe the free space on the system drive. However, that particular task is currently one of Eraser's stability weak spots; you might want to disable the erasing of cluster tips, which is what gives most trouble. With that done, Eraser can deal with any non-system partitions.
Is formatting and free space wipe a substitute for that?
Yes, but only for a non-system drive.
DBAN is showing PCI (sysfs) so asked whether is can substitute it with eraser.. More details of the DBAN isssue is in the DBAN sub forum in this site
I have responded there, probably not particularly helpfully.