I was recently given an iMac from a job I worked at as they had upgraded to a newer model.
I'm wanting to restore it so I can set it up for my own use.

The only downside to the Mac is that it has several user accounts (2 of which are password protected...the people who set these up no longer worked for the company thus taking their passwords with them) also, the Mac is partitioned and has Windows installed as well(it too with several users on it).

Whats the best way to reset the Mac to it's original state with no partition and no user accounts...or is that even possible?

It's an iMac with OS X
Specs are:
27-inch screen
3.1GHz Quad-Core Intel Core i5
4GB memory
1TB hard drive
8x SuperDrive (DVD±R DL/DVD±RW/CD-RW)
AMD Radeon HD 6970M graphics with 1GB memory

Built-in FaceTime HD camera

Thanks guys!