There may be slight difference between individual PC brands, but no computer is completely noise free. Everybody who claims "My PC is noisefree" has not looked close enough.
The main sources for PC noise are switched power supplies in the computer itself and its peripheral devices (like monitors) and noise coming from data cables (mainly USB). The noise can intrude through the antenna, the coaxial cable and through ground loops, sometimes by direct radiation into unshielded components.
1. Guys have already started to filter their USB connections for their RSPs (and similar) by using USB EMI filters like this one
and to feed the receiver not from the 5 V USB rail, but from external linear power supplies. I have heard that some of them managed to bring down the noise level in USB controlled receivers by 10 dB when doing so. Of course using high quality, well shielded USB cables is always a must.
2. Very often the power lines of PCs and monitors need extra line filters to hold back the switching power supply noise. Connecting the metal cases or ground plates of PCs to a good ground (earth) also helps.
3. Additionally you have to place the antenna as far away as possible from the PCs and use high quality shielded coaxial cable. Common mode chokes in the antenna cables also do a good job.
I myself found that the most stubborn PC noise source is the switching frequency of power supplies around 60 kHz (50... 100 kHz depending on model). Although applying all sorts of coutermeasures (like those described above) I never completely get rid of that wobbling noise around appr. 60 kHz in the VLF area, while its harmonics are not a big problem.
But what do all our efforts help when our beloved neighbours have electronic devices in their houses which have a poor EMI suppression, very often tolerated by lax legal regulations and standards. Or when they use devices which produce high broadband noise levels by design, legally approved, like these stupid powerline modems (PLC, PLT, powerLAN...).