What's the Best Way To Move To A New HDD

[ReptilianPeon]

I have quite the quandary. I'm undecided as to which option is the best. Here's the situation:

I have a PC which I purchased almost five years ago now. I want to get a new HDD before it's too late as I fear it may or may not have long left (I use several external HDDs for back-up already). The HDD came with OS pre-installed. We (meaning my father) have an adapter which can turn internal HDDs into an external storage device if need-be (presumably by means of USB) and discs with copies of the Windows OS on them. Do I either:

1. Purchase the new HDD and attach it as external storage first to install the OS (i.e. Windows for now) from a CD or download - whilst the old HDD is still in the PC.

OR:

2. Remove the old HDD first, add the new HDD in and use the disc drive to install OS that way. Afterwards I back up using the aforementioned appendage.

OR:

3. Buy a HDD with an appropriate OS already on it (if possible) and back-up as required once the new HDD is in the PC.

I hope that made sense. Thanks in advance. I'm still rather cautious when it comes to operating on PCs.