Lost OEM Windows Disc...

Hey, guys.

I've owned an Alienware desktop computer from new for the past almost-five years, and I've been performing clean OS installs each year or so in order to keep the computer running smoothly. However, I misplaced the disc for installing the OS the last time I did it, and so I tried installing the OS using another disc that I have - but, because it's supposed to be done with an OEM disc, it's now saying I'm not running genuine Windows.

The operating system I have is Windows 7 Home Premium 64-bit, and I was wondering if there's any way at all that I can either get some replacement discs or download a Dell/Alienware image from somewhere (legally, of course - I'm not asking for illegal methods!)?

Especially since Windows 10 is coming out in a week and is going to cost £100 to upgrade if I don't get this sorted, I'd quite like it sorting, if possible, please!

Thanks

Oli

PS: I'm out of warranty - and I found this out when I tried to order some replacement discs online.