Old HDD in New PC Build

Hello, I recently built a new computer system that contains my old HDD, which I transferred from my previous PC. The old copy of Windows 7 that was already installed didn't work in the new PC, as expected. Instead, I have a new copy of Windows 7 available that I need to get on the HDD. But I also need to deal with the old Windows copy.

I've already transferred all of my personal files from the old copy onto a portable HDD for safe keeping. I also went ahead and created a bootable Linux CD so I can utilize some sort of OS on the new PC while I'm figuring this out. What I plan on doing is reinstalling the new copy of Windows 7 onto the PC's HDD to be used in the new computer. I was able to acquire the new Windows 7 version for a really cheap price from my college.

So now I either have to format my HDD so that the new Windows 7 can work properly while keeping the old/useless version out of the way, or wipe the old Windows 7 from the HDD completely. Should I keep the old Windows 7 on the HDD and just never use it, or would that conflict too much with the partition used by the new version? If they do conflict, how should I go about removing the old Windows 7 partition?

P.S. - dealing with the HDD and OS seems to be one of the most annoying parts of a PC.

Yeah I agree it's a total pain. I'm not the best tech here certainly but I would say go ahead and get rid of it. I tried just ignoring Vista when i was upgrading to Windows 7 finally and it did not work out for me. Doesn't just installing Windows 7 by inserting the CD offer you to do a clean install? That's what I did I think, but of course I had functional Vista to do it from I don't know how it will work for your jumbled system.