Removing operating system

[46 the goat]

Hello everyone, this may be a stupid question but I'm a nube.
I have just finished my first build and have loaded windows7 64b on a new WD 640Gb 6Gb HDD. I now want to remove it and then load it again ( don't ask why, would take too long to explain). My question is how can I re-format the drive, when I click on the format option in the drop down menu for the HDD it says " you cannot format this volume as it contains the version of windows you are using"
How do I make the HDD like it was new again?
Thanks for your help
46 fan

 

[46 the goat]

Thanks guys for your help. before I loaded the operating system for the first time I noted that there were 14Gb used on the HDD, now since I have loaded it again there is 21Gb used. Have I done something wrong? Is there any way of empying the HDD completely and starting from scratch?
thanks
46 fan

 

[46 the goat]

Thanks for all the help, deleted windows old and now 16Gb used on HDD, does this sound OK and should I stop worrying about it. No programs in the P/C at the moment, but dektop seems to be working as it should.
46 fan