jsmithepa is right. Even more importantly, you have made MAJOR changes to both the CPU and the graphics system, and I'll bet everything else! You MUST do a fresh Install of Windows. So it is a good thing you have an unused Install of Win 7 available, AND you already have backed up your stuff to somewhere else. Just check to be SURE that your copies or backups (whatever) CAN be read cleanly so you will be able to use them.
Now, the simple process, as Ephemerally suggested, is to do a normal Win 7 Install from the CD BUT at the very first step you need to Delete all existing Partitions on the HDD so it is empty. From then on the Install process will create a new Partition and Format it cleanly for you.
HOWEVER, I like to take an extra step with older used HDD's. This takes more time and is NOT necessary - just extra caution. I like to Zero Fill that old drive before doing the Install. Why? A Zero Fill operation writes zeroes to EVERY sector of the unit, which wipes it clean. But more importantly, that also triggers a self-testing process that is done internally in the HDD unit itself - Windows does not even know about this testing. As the Sectors ALL are written to, the drive also reads them back looking for weak signals or outright errors. If it finds any, it replaces that weak Sector with a known-good one from a semi-hidden stock of spares and marks that weak sector so it is never used again. The final result of this operation is that the HDD appears to the outside world (read: Windows) to be completely flawless and containing NO data. Then you can do your normal Install, but you won't have to Delete any old Partitions because there are none.
This self-check process is a behind-the scenes thing that goes on all the time as a HDD is used. As you might anticipate, eventually the stock of known-good spares gets low. All this is part of the self-monitoring process called SMART, and that can send out a warning message when too many bad Sectors have been replaced. If your computer's BIOS is set to check for SMART messages at boot time you will see any such warnings. Or, if you use HDD diagnostic software utilities, most will show you the SMART messages. IF you ever get a SMART warning about a high Sector Replacement Count, it means two things: (a) the stock of good spares is getting smaller, and you don't want to go too much further and run out; and, (b) since so many bad Sectors have been corrected, more will likely happen and maybe faster. So the smart move at that point is to buy a replacement HDD and clone your old unit to the new one while it is still able to give you all your data without errors.
My preference is to do the Zero Fill and then check the SMART data to be sure there is no warning about high replaced Sector count. THEN you can be confident the used HDD is still in good condition and go ahead and use it. But as I said, it's not necessary - just my preference when re-using an older HDD.
To get a Zero Fill utility I can suggest two sources. You mentioned DBAN, and that's certainly able to do that. Another possibility is to download the free HDD Diagnostic Utility package from the company that made your old HDD. If it's from WD, get their DataLifeguard utility. If it's from Seagate, get their Seatools. My own preference with these utilities is to download the "for DOS" version. It is actually an .iso image of a CD you make. You then need some CD "burning" software like Nero that is able to burn the .iso file onto a CD. Once you have that you're set. The huge advantage of this CD is that you can boot and run from it with NO working HDD in your system - you do NOT run the CD under Windows or any other OS. It contains its own OS, and it is bootable.
To use that diagnostic CD you burned, you place it in your optical drive and boot directly into BIOS Setup to make sure that your Boot Priority Sequence is set to boot first from the optical unit, and second from the HDD. If you have to make a change to set this up, remember to SAVE and EXIT. The machine will boot from the diagnostic CD and give you info (like current SMART data, drive ID, etc) and offer a series of tests and processes you can run. Most tests will not damage any data on your HDD. Those that processes that WILL destroy data (like the Zero Fill!) will warn you first and ask permission to proceed. In your case, OP, you probably don't care about the warnings for this purpose. If you do a Zero Fill it takes a long time. After all, it is writing to (and reading back) EVERY Sector of the unit. So just let it run and do something else for a few hours.