Hp Laptop — I'm complaining about the problem in operating system of hp

Address:

West Godavari, Andhra Pradesh

I have purchased hp laptop on 19.10.2015 at "jnana prasunamba computers", Bhimavaram.
laptop model :
intel core i3,
5th generation,
4gb ram and screen is 15.6 inches.
PROBLEM:
at time of purchasing the hp laptop has windows version of 8.1, it has problem at the time of starting..it takes about 15min time to start up.so, finally we have uploaded windows 7.i would like to know the reason for this fault why this lapy is unable to support windows 8.1 version.