It is definitely possible. I also have a late 2009 iMac (although the 21.5 inch model) and I can give you some help.
You can use Boot Camp Assistant to partition your hard drive and create a dualboot Windows and Mac OS X configuration in no time. Windows 7 is the recommended version, but if you prefer you can also install 8 or 10. (Only Windows 7 drivers are available though).
Boot Camp assistant is an app preinstalled on your mac - its there already, no need to download anything. Connect a flashdrive and it will copy the drivers there. Insert your Windows install DVD and the assistant will reboot your mac and you can install windows as usual. Then you can install the drivers.
Windows will now become the default boot option. You can change this from System preferences in Mac OS X or control panel in Windows. There should be startup disk or boot camp preference panel added respectively.
Unlike on a PC there will be no countdown on boot up, the default will boot automatically, every time.
There's an override of course: If you don't want to change the default setting just make a one-time override, you can hold down the ALT button during bootup to enter the boot menu where you can choose to start OSX or windows.
You asked if it is possible to erase Mac and have Windows installed only.
Yes it is possible, although I wouldnt recommend it. (No issues but do you really want to get rid of OSX? You might find a use for it later and it will hard to reinstall it into a dualboot configuration later with windows already installed. Instead I would just suggest shrinking the Mac partition for say, 40gb so it wont bother you at all and leaving windows be the default boot option so you'll never see mac os x, unless you want to)
But, if you still want to get rid of it, then just skip all thats above, insert your windows dvd and reboot, press alt during bootup and select the cd icon.
Then just delete all the partitions and install windows, click create new partition, agree to create additional ones if asked and thats it.
Still make sure to copy drivers from within osx BEFORE deleting it.
Note again, I do not recommend removing OSX.