Changing the fan, moving the computer to another room and using keyboard, monitor and mouse extension cables would all be good solutions, as would the acoustic treatment you can't do.
However, for some quick and dirty improvement, make the physics of acoustics...
-Every time you double the distance from a sound source, you quarter the amount of sound you're hearing...so, if you can move twice as far from the computer, you'll only have a quarter the fan noise.
-You don't mention how far the mic is from your sound sources but, just as with the first point, if you can halve the distance from mic to to your mouth or guitar, you'll have four times as much level compared to the background noise.
-Your SM57 is a directional mic...it picks up strongly directly in front and has very little pickup directly behind. Use this and arrange your room so when the mic is pointing straight at the guitar or your mouth, it's pointing directly away from the fan.
-Finally, it's not acoustic treatment but if you can hang something soft (duvet, movers blanket, etc) behind you when you record, it'll stop some of sound from behind the mic being reflected into the sensitive end.