Your PC has a built-in DAC. It's in the audio chip which is soldered to the main circuit board (called the motherboard) of the computer.
You can use the colored audio-out minijacks on the rear panel of the PC to connect almost any speakers, stereo amplifier or A/V receiver. The audio coming out of those jacks has already been converted from digital to analog by the PC's built-in DAC.
Alternatively, you may use the optical digital audio-out jack located close to the colored audio jacks. This connection is still in digital form. You may connect it to any separate DAC, speakers, amplifier or A/V receiver which has the same optical digital type port. Obviously, the PC's built-in DAC is not converting the audio to analog in this case.
My suggestion would be NOT to spend any money on a separate DAC at this time, but rather to do a lot of listening for several weeks or months to see if you're satisfied. After you become very familiar with the sound quality you're in a better position to judge what improvement, if any, you can achieve from additional spending on a separate DAC, or upgraded speakers/amplifier/receiver.
One of my PCs has Creative Gigaworks T40 Mk.II speakers connected to the analog audio-out jacks on the rear of the PC.. They are a nice sounding pair of powered computer speakers for around $130-$150 (no DAC). Being very familiar with the sound i introduced a separate DAC between the PC and the speakers using optical digital cable to the DAC, then stereo RCA-to-minjack cable from the DAC to the speakers. The DAC is called a Bifrost and costs around $349 or $399. The Bifrost offered a modest improvement in midrange detail and high frequency precision. The improvement was definitely not sufficient to justify the cost of the DAC to go with those speakers.
Our living room A/V system is PC-based and has the same Realtek ACL1150 audio chip as the other PC. This time it connects to a Rotel stereo amplifier and Magnepan speakers, a combination which is way better sounding than the Creative Gigaworks. In this system inserting the Bifrost DAC between the PC and the amplifier makes a considerable difference, thus easily justifying its inclusion in the system. Having listened to favorite recordings ripped from CDs to the PC in .wav format (no compression of any kind) many, many times prior to adding the DAC, the improvement was clearly audible right away. After a few weeks of listening i calibrated the volume setting of each input on the amplifier to be the same at 1KHz then asked my GF to switch between inputs while i listened to several favorite classical, rock and operatic pieces. We repeated the comparison the following week with a different selection of music. With the Bifrost engaged in a Haydn string quartet (digital recording) the instruments were more easily distinguishable one from another with the very highest notes having more shimmer. In Il Barbiere de Sevilla (analog recording) Maria Callas' high Cs were spine-tingling with no shrillness. In Joe Cocker's Sheffield Steel album his raspy voice was a more front and center and the cymbals had more precision (more crash, less splash so to speak). In these comparative listening sessions i was almost perfect in determining whether it was the Bifrost or the PC's built-in DAC with one glaring exception. Santana's Abraxas album is an old favorite, but i was unable to distinguish which DAC was in use on any of the tracks. Yes, they sounded slightly different but i had no preference and could not correctly state which was which.
Your music, your room, your equipment, your ears and your personal preferences might well come to a different conclusion. I say trust your ears and go with whatever gives you the most musical enjoyment.