Video slows down after playing for a while on my HP 15-D041TU (no heat problem as well)

I've bought this laptop brand new from the shop, it came with freedos and i installed windows 7 64bit and service pack two as well then i installed VLC Player, SM Player, & Daum Pot Player just to use on my day to day video watching.  Well everything works perfectly fine except that when i play the video for a while, it started to slow down, it works nicely otherwise. It starts somewhere around 30 minute and beyond. I tried with all players, and this happens with BlueRay Rips i have which is HD video. Whats happening people? is this computer not powerful enough to play some HD Movie?. If it so then i am really in big trouble because the main reason for me to buy this was to watch movies and browse web. If i can't do one of them on this it really is bad for me . What causes this? anyone has this problem?. I watch movies plugged to AC all the time and Balance power profile. I also watch them in standard Settings of the player. The machine has 2GB Ram and Celeron N2810 Processor. Is this a problem with windows 7?