Thanks for your answers! You are all very helpful, even guessing a bit since I wasn't clear enough.
I have to say, the suggestion by MadCatX "Only thing you can do is disabling all services in msconfig, terminatle all unnecessary background tasks and set LFS to realtime priority in taskmanager." is something I tried, but setting to realtime downgraded performance. Effectively, the OS needs cpu cycles to serve LFS.
I had never thought of having a computer just running LFS and nothing else ever. I wish I had enough money!!! Linux would definitely be the solution in that case.
To be more clear: what I want is all the cycles reserved for LFS *except* the OS can act, if it is to serve LFS.
I don't want this always, just when LFS.exe is active.
What I *don't* want:
-BITS
-cdrom autocheck polling
-random OS calls not related to LFS
-"other" threads executing on my cpu
-basically anything that the OS does that is not in my control that takes up CPU cycles.
So, I am installing linux.
"About that linux LFS-only distribution: Linux also uses CPU cycles as same as Windows do, but in linux is everything complicated because of WineX or Cedega."
but in Windows, everything is complicated because you don't actually have final control over your machine. I dare anyone to try, shut down as many applications and services as you like, then run filemon and regmon and tell me you have absolutely no "noise" coming from your OS
I will take the (measurable, static) slowdown from cedega over .. well just what exactly is happening on my machine anyway?