it's not free, you may be able to download it as a pirated copy and it work for a while, but one day after an update etc you might not be able to use your machine. some are leaked volume licences (a company may buy a volume licence, same code to cover 500 machines). Once microsoft know of these licence numbers, they effectivly cancel these licences and write an update forcing you to validate you copy.
I repair and sell pc's, and people always say "can't you just put a copy on" I explain that in the long run, £60 spent now over the life span of say 3 years use is good value at £20 a year. You are then legit, you can do updates and don't have the hassle or worry.