Windows 7: How can I restore my computer to factory defaults? but....

I remember a while back I got a BSOD, but I fixed it by reformatting my C drive, this didn't remove my OS or touch anything on my D drive however.
I would now like to do the same, not because I have a problem but because my computer is becoming sluggish and I haven't reformatted for a long time.
However, searching up how to reformat the C drive, without removing this OS seems literally impossible, every post I see just says the following: get your OS disk.
Despite this, I managed to get the C drive back to factory defaults while keeping the OS, all my drivers for my hardware (graphics card, sound card etc.) intact. I would like to do the same again, if it is actually possible.
This was originally a box PC so I don't have any CDs except for the one for my new graphics card which I replaced a faulty one with.

Hello Fatalen, and welcome to Seven Forums.

What brand and model number is your computer? I'll be happy to see if I can find the instructions for how to do a factory restore for it.

Acer Aspire M3870 is what the sticker says. (quite an old pc)

Here you go.

Acer Support: Frequently Asked Questions list for Recovery media and Restoring a system to factory load