I have a spare HDD that I'm thinking of hooking up to my current PC (from a PC that just broke). However, this HDD also has Windows 7 installed on it and I'm afraid it would conflict with my current setup.
So my question is; Will the HDD conflict with my current setup, since it has OS installed on it or not?
If windows 7 was installed on the spare HDD you have, you will not be able to boot windows from that HDD. The OS from your spare HDD will only boot when paired with the original motherboard used when the OS was installed on it. However, assuming it's formatted the same way, your current PC will certainly be able to use your spare HDD as extra storage space.
just format the second drive ... you will lose anything that is on it but there will be no conflict ... you would not be able to access any data anyway from the primary win 7 OS you booted from.
if there is any data you want to keep off that drive ... save it elsewhere.
Edit ... and it is possible to have multiple OSs on the same drive ... but that is not what you asked. And is more complicated.
If that is what you mean ... ask again.