don't build a box/booth, cos it'll sound like the mix is in a box/booth.
Don't box up the computer, cos it'll just overheat.
Either, buy bigger fans that can run slower, or get some water cooling.
if the fans aren't as noticeable in linux, then its safe to assume they're being controlled by your os, so either record in linux,
or see if you can download some fan control software for xp and take it from there.
If you do get em to run slower in xp, make sure you have a temperature monitor program running and keep an eye on those temps.
That said, if you're not overclocking or overstressing, you shouldn't be overheating.