NOOB ALERT: having trouble streaming films

First let me say im sorry if this has been asked and answered already but i need a definitive answer as im not 100% confident in my puny knowledge of networks & streaming etc  

So my problem is that i have all my films on my main pc upstairs, (where i work so it cant be moved) but i want to stream them downstairs to my ps3 and pc (and to my tablet too! Although that would be nice there is really no need for that too  )

I did manage to stream them but i've done a fresh install of windows 7 so its not set up as it was before, so what is the best program i.e. compatible with the most video formats and work well with ps3 and other win 7 pc's?

and how should/could i configure my pc's and routers so it goes superfast, because when it did stream it was really slow, like it would play a few seconds then buffer or whatever for a while then play another few seconds, kinda unwatchable really! but not sure why though?

Just so you know my pc with the films on is connected via ethernet cable to a router on repeater mode which is connected directly to my downstairs router (which is the one receiving internet signal from the phone line) and my downstairs pc is connected to the downstairs router directly via ethernet cable.

the slow pc to pc and pc to ps3 connection is really confusing as using the same setup with both routers etc my pc with the films on can download at at least 3Mb/s from things like steam client and the same for my pc downstairs, so i dont understand why its so slow just sending from pc to pc shouldn't it be faster as it doesn't need to use the internet just needs to send from pc directly to the other pc?

Thanks in advance

Alex