Video playing slow, audio out of sync. Windows 7. Help please!

Hello all, I've had a really annoying thing happening and am hoping someone here can help. When I try to play a movie on my computer the video is really slow and choppy, audio is out of sync and sometimes it even gets choppy like listening to a scratched CD or something. I do not have this problem with tv shows they play just fine with absolutely no issues.
All files are on my internal hard drive, I moved one to my external hard drive and played it through my blu-ray player and it worked so I know the files are not corrupt. The files I'm trying to play are mp4's
I've tried windows media player, windows media player classic-home cinema and real player. I have K-lite mega pack installed as far as codecs and its the newest version.
I recently installed windows 7 on a brand new Western digital Black 500gb hard drive, also upgraded my memory from 2x512gb sticks to 3x1gb sticks (there would be 4 but I had to rma one)
I cleaned all the dust out of my computer case as well when I was installing those components including removing the heatsink,cleaning it and reseating it using artic ceramique thermal compound.
I am running windows 7 64 bit
the computer is a Hp Pavilion A1240N specs are here
HP Pavilion a1240n Desktop PC Product Specifications | HP® Support
everything stock except for the parts I mentioned above.

Could someone please help me with this issue, its really annoying and no one else seems to know what to do.
(before anyone says it yes I know my computer is old, I cant afford a new one,also I could move the files and play them on my blu ray player but my external hdd is running out of room and I cant afford a new one)

Thanks for any help, it would be much appreciated!

Ok so I tried VLC player, it played the files and everything was in sync but it was still a bit jerky and heavily pixelated. So I'm thinking that didn't really solve it but thanks for the suggestion it was worth a try.
Next I ran the sfc/scannow and the disk check
Disk check came up fine no errors whatsoever which I'm glad being a new hard drive.
sfc/scannow came up with errors heres what it said, I also ran it several times but it wouldnt fix it.



I also have the CBS.log file that I converted into a zip folder if anyone wants to take a look at that or if it would help, cant figure out how to add it to this. I have no idea how to go about fixing it using the CBS or if it has anything to do with the problems I'm having so any help would be appreciated, Thanks again for the input so far.

Ok Ill try it again. oh yeah and yes youtube and netflix seem to play fine.

I ran that 5 more times and yeah it couldnt fix it. Do you lose anything when you do a system repair?