Hum Pauly Mate the memtest should have nothing to do with the drive as it runs off a boot disk.
May I suggest Paul you get a second drive just in case - they are cheapish now and it will always keep for a backup mate. and try installing the OS onto it via the stick ISO method.
That is you can download the OS from My Digital Life and use the activation code you have as it is tied to the motherboard not the HDD. Windows 7 ISO x86 and x64 Official Direct Download Links (Ultimate, Professional and Home Premium) « My Digital Life
< put this on a stick and boot from it with the new drive (set the BIOS first to boot from USB)
But run the memtest first if you can otherwise all it should cost you is a new HDD
Personally if it were mine I would get a SSD and have a regualr HDD spinner for datas backup - nice if you want to go fast.