Windows OEM

Hi, i bought a prebuilt pc from best buy and am now looking into my own build. As the current OS On my hard drive is windows OEM Is there any (LEGAL) way to puti t on my new system? I'm using the same hard drive from this computer on the new build

whats wrong with just upgrading this pre-built computer? with say a graphics card?

Do you still have the windows serial key? If so, you can download an ISO that correlates to your serial key, burn it to a disk and install. This is perfectly legal, just make sure not to download a cracked version or anything. 

that's not legal at all. lol.

my bad it seems they have changed the license in windows 8. so it is perhaps legal.