My PC knowledge ends about 7 or 8 years ago, so ideas from me may be a bit dated...
But whilst we still have many design artifacts of the X86 based PC layout, I suppose some of this is relevant.
Somewhere I read that shared USB devices could cause noise across the serial bus.
I think it was in the Tascam US366 manual, that said not to use any other USB devices at the same time as the US366.
But,I broke all the rules anyway, just to see how far I could get away with it.
It's not like I'm printing while I record or anything...
I use a wireless desktop, via USB as the keyboard in my laptop is on the blink and I can't be bothered repairing it.
If I move the mouse around whilst in record, I can hear a very quiet noise, but I don't use the mouse when recording, my hands are full with guitar.
I've had no noise issues with noise with that US366 and a stock WIN 7, 4GB RAM, ACER laptop, no upgrades.
Tascam/Teac have probably 40+ years experience in preamp design, so I have no issues with noise with the US366.
I did mean to get around to ripping the optical DVD drive out of my laptop and putting in a 2nd HDD.
(Possibly making C drive into SSHD and shifting the C drive into D position, in the old DVD slot)
This would dedicate C:/ drive to run programs & DAW software duties, and leave D:/ drive as the target directory for recorded/recording audio data.
This always worked well on the old IDE drive systems for me, designate the drives, take the load off the processor, with RAM, dedicated video & sound cards and leave it plenty of room, not to stress, but that was my knowledge from 1990s PC architecture.
A standard laptop DVD drive can usually be removed and put into an external case for under $10 (on amazon) with a SATA to USB adapter, so one is not bereft of optical drives, if required. Then a 2nd HDD put in a carrier tray (again, on Amazon, for like $7) in it's place, giving the 2 HDD setup I like for recording audio.
In the past, I've always found HDD noise through the system could be pretty loud, maybe not audibly, but measurably. certainly above any other specified noise floor for any one part of the signal path through a PC. Some of my more knowledgeable tech friends have shown me my noise issues on oscilloscopes before and helped work out where they came from.
PCs are generally noisy anyway (relatively speaking), with switch-mode power supplies, which are renowned for their noise characteristics, unless manufactured to a much higher spec than domestic PC use. We used to make switch-mode PSU's for satellite amplifiers and we made those quiet, but it was not cheap.
I use toroidal transformers in my PSUs of my amps, like ROTEL sometimes do, they can be really quiet.
I did notice with differing brands and designs of PSU units for desktop PCs. that I got less noise from some of them.
I've not yet tried a Solid State HDD, but rumor has it there is a lot less noise with no HDD motors to interfere.
A lot of noise comes back to good old Earthing/Grounding checks and multimeter work.
If there is an Earth loop, or a lifted Earth somewhere, it will dutifully act as an antenna and probably pickup any internal RF, from any number of places.
I have a little RF probe tool left from telecom tech days (one half of an F-Set), I can position it on any part of a PC and hear from it's speaker, what each type of noise sounds like, coming from each component.
I can hear HDD noise with it and trace it through cables, I can hear the RAM screaming away, and almost any other component that emits some kind of RF and most do.
If I have a noise problem, I always check for solid Earthing, good connections to chassis, check the PCB rail around the outside edge, check (or mod) all the filter caps in the PSU, sometimes on the motherboard too and generally tidy up, sometimes I can rid a system of noise, sometimes not.
It may be the Steinberg card is the problem, I have no familiarity with them, but as a general rule, when I buy electronics I buy from the few principal manufacturers that have experience in that area. I'm not sure if Steinberg have any credentials long term in the electronics manufacturing industry, or who makes their stuff, no idea, so I can't comment. I went for Tascam, because I knew that I would probably get low noise preamps and I did.
I still use 2 preamps in the front end of a 1970s reel to reel tape machine sometimes, they are quiet and have a nice sound.
I've heard far better preamps, but not for the $$$ of the US366, given it's ability to easily handle 4 incoming tracks and send them all to C drive, at 192K, with a 32 bit float.
I've put it near tube amps, Variac, PSUs, wireless cards, mobile phone gprs, wcdma, 3g, 4g, anything short of a power drill and the US366 won't pick it up, even then it must be through the phantom power circuit, as it's pretty lightweight, running a cheap internal step up transformer from a 5VDC USB power feed and all...
The power drill from a neighbours house does come out of my guitar amp. but that's an issue with pre-historic house wiring...
I've never had even a single glitch or latency issue with the US 366, not one.
Touch wood it stays that way...
Cheers.