MKV and MP4 files no longer play

I did a fresh install of K-lite codec pack (full, mega wouldn't install...) and now when I'm playing MKVs/MP4s it'll consistently (at the same points in time in given files) fast forward to the end of the file. It's made video watching impossible on my PC in all media players.

What kind of error did the mega installer give?

You are the only one with the strange problem, so it is caused by something specific on your system. Are you experiencing any problems any problems with other applications, or instability in Windows?

My first suggestion would be to scan your harddrive for faults. Right-click on your harddrive in Explorer, go to Properties, then the Tools tab. Click on Error-Checking. Enable the option to scan for Bad Sectors. The start the scan. Depending on the size of your harrdrive this can take a while, up to an hour or even a bit longer.

It was the LAV video splitter. I installed HAAVI and the problem went away.

LAV Splitter is working fine for me and everyone else. I would suggest re-installing the codec pack to see if it works again now.