Actually, its ALL alcohol. I know some people have had issues with using alcohol and/or Windex, but for the most part you shouldn't.
[warning: science geek stuff ahead]
Key ingredients in Windex are isopropyl alcohol and a bit of ammonia (small amounts of other surfactants and solvents). Ammonia is a good window grime cleaner but its the booze that cuts our acrylic paint. Trouble is, there's not much of it, ~30% (depending on brand/formula/who you ask) so you are buying a lot of water and ending up with a relatively weak airbrush cleaning solution.
Better (or at least stronger for you money) is good old rubbing alcohol which is 70% isopropyl alcohol and water (or 99% if you get the hard stuff from behind the pharmacists counter). I use it for initial flushing of the gun, always finish with Vallejo Airbrush Cleaner though. NEVER had a problem.
Key ingredient in Vallejo Airbrush Cleaner is butyl glycol (got that strait from Vallejo), which is a type of alcohol (similar, but not identical, to what is in Tamiya acrylic paint and thinner). Works extremely well cutting Vallejo's paint formulas, even totally dry paint. I dont know the concentration but one whiff of it suggests its a pretty decent, plus I believe it also contains detergents (as they claim it "lubricates" which is something alcohols don't really do). Bottom line is it WORKS! Definitely buy yourself a big bottle (or two) MrNatrual. You wont regret it.
Vallejo recommends using a 50/50 mix of their cleaner and water for initial "flushing" of the airbrush (I think as a cost saving measure), always finishing with pure cleaner though (tried that but prefer using the rubbing alcohol for that initial flushing etc, always finishing with pure cleaner).
Vallejo also recommends a 50/50 mix of their cleaner and alcohol as a soak bath but I just use pure cleaner if I ever need to soak a paint tip or nozzle.