Using a old Hard drive with new system no os

[seperation]

I currently ordered everything so far except a hard drive and a operating system for my build, I have a old computer with a hard drive and i was wondering if i could use this with my new system and will it contain the operating system still? or can i wipe everything clean and use a free operating system till the new hard drive and vista 64 bit to come in a days/weeks.

 

[0mg_1ts_m3]

it will still have everything it used to on it.

 

[seperation]

So it will still have my operating system? i can boot up with it?