Chasing noise out of a studio needs to be a step by step controlled process.
1. Unplug everything from the PC. That means everything you can get away with unplugging, not just the audio ins and outs to the soundcard, but network, joystick, peripherals, everything not needed to run the PC and record the soundcard's baseline noise. Record an audio track, save the file, and use that for your baseline noise level. Right off the bat, if you have too much noise at this point, it's as issue with your PC. Could be inside the PC, could be leakage from outside, but it's beyond what I'm prepared to address here. If you try this with just about any Soundblaster or Creative card, you are liable to be surprised when you see how noisy they are, especially the older ones.
2. Next, plug in the rest of the PC peripherals you plan to use, USB devices, joystick, everything except the audio cables and watch the meters. You'll need to have an audio track armed for recording. Compare the noise levels to your baseline. If there's any jump in noise levels note which cable or device caused it and write it down. Better yet, get rid of that device.
3. Plug in just your audio in cables, as few as you need to get a real signal. Leave the other end of the cable unplugged,you just want to measure the cable's noise here. (In some cases, an unplugged cable can pick up hum, so if you get a big noise spike here, don't toss the cable yet. Go ahead and connect the other end and see if the noise goes away.)
4. Keep connecting input and output cables to the PC until they're all in. Watch your midi cables too, not just audio cables. Big jumps in the noise level can be a bad cable, they can also be a cable that's running in the wrong place. I run data and power cables under the desktop, and audio cables across the top of the bridge (shelf above the desk). Risers (vertical cables runs) for audio are to the left, risers for data and power are to the right. The PC monitor is a big noise sump, keep your audio cables away from it, far as you can. The key is to keep the two types as far apart as possible. if they have to cross, do so at a right angle. Even a few inches of physical seperation can make a big difference. The inverse square law is at work here.
5. Once the cables are all connected, start and complete plugging in the next layer of devices, one at a time, comparing the noise levels after each step to your baseline. Be sure your gain levels are at or above where you'd use them while working, all the way through the chain. Note that you can't change a level after testing that device. Otherwise you are comparing apples to oranges. Keep working through the process one step at a time. Every so often, don't just listen for more noise, look at the numbers on the meters, and compare them to your reference file. Noise has a way of creeping up on you, and the numbers can be the only way to see the total effect.
Write down each device that adds noise. Unless you win the lottery, replacing them all at once is improbable, and by the time you can afford replacement, you can easily forget what the noisiest devices are.
You're going to see lower noise with balanced cables as opposed to unbalanced ones, but you have to work with the devices you have. As you expand and upgrade your studio, keep track of your big noisemakers and works towards getting them balanced first.
Short cables will pick up less noise than long ones.
You can avoid a lot of ground loop noise and hum if you daisy chain power strips from one plug on one outlet to run the whole studio. If you only have one spike protector, it needs to go closest to the wall. Make sure that the circuit will handle the load. Small home studios ought not to tax a 20 amp circuit, but you might with a 15 amp circuit. If setting up a semi-pro or pro facility, take the time, look at the UL labels and do the math, or get a licensed electrician to do it for you. Screw up here and your whole building is toast. Not to mention all your toys and whoever lives there.
Look at what else is on that circuit too. Light dimmers and motors make noise. Keep them off your studio circuit. Keep them as far away from the room as possible, even if they're on seperate circuits, especially dimmers. Big power devices put spikes on the line when they come on and off. Microwave ovens, hairdryers, irons, toaster ovens, if it glows, it's soaking up gobs of current.
With audio devices, I'd look especially hard at the meters when adding synthesizers and samplers, guitar and bass amps, EQ's, mikes with unbalanced or long cables, and distortion type high gain direct boxes.
That ought to get you started, if you get into and come up with more questions, feel free to ask.