An important thing to keep in mind is that a designed-for-music audio interface literally is the audio recorder. That is what makes the audio recordings. Your PC is only supposed to provide the user interface and file storage for your audio interface. The speakers provide the output for your audio interface. Microphones and guitars provide the input for your audio interface.
If you imagine a standalone digital mixer, the knobs and sliders are moved from there onto a computer screen, and what you are left with is some audio inputs and outputs — that is the audio interface. The audio inputs and outputs move onto the computer by plugging them in via USB or FireWire or Thunderbolt. Now your PC is a digital mixer. The built-in audio from your PC continues to provide a way for your PC to beep at you. That is what the built-in audio is made for. It is most certainly not made for audio recording and production.
The audio interface is the most fundamental piece of gear in your studio, same as an analog mixer was the most fundamental piece of gear in an analog studio. I recommend you start with the audio interface and build out from there. Figure out what are the fewest inputs and outputs you can get by with (e.g. 2x2 might be just fine for you) and then get the best quality made-for-music (not speech, not gaming) audio interface you can afford.
I use all Apogee gear. You should check them out at least, because it is all great stuff, all made for music, all 24/96, a whole range of products from smallest to largest, and they sound great. However, I don’t know if they work with Windows. But it can be educational to look at their product lineup and figure out which one of their many options would be best for your needs based on the size of the interface and the number of inputs and outputs, and then you may move on from there to another manufacturer that has a competing product that is lower-priced or suits your other gear better. For example, if you determine Duet is ideal for you but too expensive, you can then identify a Duet-competitor from another manufacturer that has similar features but is cheaper, or you might buy a used Duet on eBay to get it cheaper.
The key thing is to get an interface that is the right size, price, and number of inputs that is right for you. Don’t get an 8x8 interface and use only the first 2 inputs and outputs all the time because you are better off getting a higher-quality and smaller 2x2 interface in that case. It will sound better, be more portable, and maximize your dollar. If you are going to plug a condenser microphone in to your interface and record singing, you’ll want an interface that provides phantom power.
If you are paying about US$300 each for your HS7 monitor speakers, the audio interface that matches those speakers should be US$350–$500. Anything less and it may not have balanced outputs that the HS7’s want. Anything less and the drivers may not be reliable and you’ll get glitches and crashes as you work. Again, the audio interface is the recorder.
Keep in mind that the HS7’s are not made to sound good, they are made to sound transparent and show you the flaws in your music, and then you are meant to do a bunch of work on your music until it does sound good through the HS7’s, and then ideally it should sound great everywhere else. As long as you are aware of that.
If you are recording voices (whether singing or rapping or voice-based samples) you will want a decent microphone. I use AUDIX because they sound great with my voice and they are outrageously rugged and I’m mobile all the time. You’ll want to find a microphone that suits your unique needs in that same way. Typically you will want a condenser microphone for studio use, and a pop screen, which is like $19 and worth every penny. You should probably spend US$200 on a condenser microphone. For example, the AUDIX CX-112. You don’t have to spend more than that to get a decent microphone, but if you spend less, the quality will fall off very quickly, and also the reliability, and you may end up buying two $100 bad-sounding microphones over the next couple of years instead of one good-sounding $200 microphone that could last you 10 years or more.
A big thing with gear is that if you buy something that has good quality and suits your needs, it will pay for itself right away and last a long time and be reliable when you are using it so that you can just focus on making music. If you buy something that is low quality and breaks right away, or simply doesn’t suit your needs, then that gear not only destroys your budget, it also distracts from your music-making time, focus, and quality. So try to have as little gear as possible with the highest-quality that you can afford. Before you buy something, make sure you really need that new piece of gear and that it really suits your setup and that you will get some very real result from it.
One last thing: get good cables. Cheap no-name cables are not only unreliable, they are often made with harmful chemicals that can affect your health and poison the environment when you inevitably dispose of them after they break way too soon. You don’t have to spend a lot, but you should not spend a really small amount. Get the cheapest name-brand cable and it will be 2x the price of the no-name cable but it will be 10–100x the quality of the no-name cable. For example, get the cheapest Monster StudioLink XLR mic cable or 1/4-inch balanced line cable for your speakers and they will last you for years and years while giving you reliable performance. Your studio is only as good as your cables.