Best way to see if you need the Modi 2 Über is to play the highest quality song that you have on your PC. Then listen carefully, and in the quiet sections of the song, see if you can here noise. If you can, then its a fair bet that a DAC will be of some use too you.
I know from my 27" iMac, when I played Neil Diamonds Hot August Night, when I just had the Magni 2 Über hooked in, I noticed a little background noise, and when I finally received my Modi 2 Über, it was less so, and I had to actually listen to hear any, and it wasn't there.
Put it this way, in your PC there is a DAC, and because the PC maker, made it to compute 1's and 0's and not really sound, it would be a fair bet that it has low end components. Schiit make a DAC for 1 purpose and thats to convert Digital to Analogue, so their components are of higher quality, and for my reasoning, a better DAC than what is in your PC . . . .
Getting a DAC though, isn't going to suddenly make your HP's 'Blow You Away', but at the end of the day, I bought the Schiit Stack, to get the best sound possible to run through my Grado SR325's and soon GS1000's