how do i fix the problems with downloading and youtube?

i recently got a Sony Vaio SVE1512W1ESI.CEK which runs on an intel i5 processor and comes with windows 8, everything seemed to be ok at first but then as i tried to download firefox (the first thing i'd usually do with a new computer) using internet explorer i encountered a problem, firefox downloaded up to 99% then stopped and went no further, i tried the download a further 4 times with the same result so i thought it must just be some sort of problem with the firefox download. i then tried downloading itunes multiple times but the same thing happened again, i tried steam and yet again the same thing happened. i've restarted my laptop over and over again but that did nothing.
the second problem i've encountered is with watching youtube videos or any video on the internet, with youtube (youtube is the example because there's more control and it's easier to explain) if i click on a video the standard definition version takes either takes ages to buffer then plays or (this is what happens most of the time) just buffers and buffers endlessly and never plays. watching HD videos is even worse, it plays the first couple of seconds of the HD video then the sound disappears and the video screen goes green and nothing can be done from then on, nothing i've tried has worked, i read somewhere about right clicking on the video then disabling hardware acceleration but that does nothing.

i hope there is some way of fixing these problems as there is no point in a laptop which you can't download programmes for or watch youtube videos on so i would alternatively have to phone the sony helpline or return the laptop to sony from whom i bought it from directly which i really don't want to have to do.

any help would be greatly appreciated