How can I turn my current PC into a NAS device without losing the PC

"NAS" (Network-Attached Storage) is generally used to refer specifically to a discrete machine which does nothing but serve storage space across a network, but that's not really what you're looking for; what you seem to be after is simply the ability to serve storage space to network clients, specifically without wanting a separate machine doing the job. The good news there is, that's quite easy to accomplish!

Considering that Linux and Mac OS X both have SMB (Samba) clients, all you really need to do here is set up the additional storage as a RAID and share the RAID volume via Windows File Sharing. Once that's done, your NAS clients will be able to access the share in order to read/write files and back up to it, and you'll still have the existing Windows install on the machine that's serving as your NAS.

You don't mention in the question what version of Windows you're running. Windows 7 and above have built-in software RAID capabilities accessible via the Disk Management console, and How-To Geek offers a good overview of configuring Windows software RAID; for Windows XP and earlier, you may need to purchase a third-party product such as Partition Magic or Acronis True Image. Of course, if your system supports hardware RAID (as most newer motherboards will), then you have that option as well.

If you need some detailed advice on how to set up a file share in Windows, just comment with your Windows version (and, ideally, also edit it into your question), and I'll update my answer with step-by-step info.