pc computer used under $500.00 for music only

Depending on the horsepower you need you can get a nice Atom based PC for less than $500. Zotac makes nice ones. Atom processors with hardware acceleration can easily handle 1080p movies let alone high res FLAC files. I personally use two of them for my audio though I will be making a CAPS in the near future for my main listening room where I want the best sound quality.

Mojo Audio modded Mac-Mini.
Thrilled with ours!!

The PC is easy with that budget. I got an Acer Aspire, put in an SSD 250gb drive and downgraded to windows 7. Final move was to buy Jriver. There went $500.

However, I spent almost as much on a DAC with USB input. Your system will only be as good as the signal coming out of the PC. If you have a desktop, you could install a good soundcard. However, I really don't think there is a soundcard I would buy over my Schiit Bifrost.

The other option is to get an SPIDF adaptor (Music Fidelity V Link) to convert USB to digital and then go into your digital input in your system (if you have one)

Asus EEE Box with the Atom processor. About the size of a router and no fan. Strip down the software to minimum and install Fidelizer.

I am using an Asus 14 inch laptop with an Intel i3 processor, 4gb ram and a 1tb hard drive. I am using a Western Digital 2TB usb3 hard drive for all my music files. The laptop supports usb3 and is Windows 7. I use J River Media Center 19. Total outlay was around $500 new.

I turned off all the services that I didn't need and have very minimal software on it. It runs like a dream and I can access over 500 cd's at present in flac. I also use the app j remote to use my iPad and iPhone as a remote control.