What codec do you need and how do you know you need it?
Windows 8 comes loaded with some basic codecs. And Premiere Elements adds codecs for most camcorders and phones. If you've installed Quicktime from Apple, per the program's instructions, you'll get even more.
It's rare you should need to manually install a rogue codec on your own.
Please do NOT install a "codec pack" since there have been some reports of hard drives needing to be reformatted after doing such
Report back here with the DETAILS of the video file that is giving you problems and somebody may be able to help
Codec & Format information, with 2 links inside for you to read http://forums.adobe.com/thread/1270588
Report back with the codec details of your file, use the programs below... A screen shot works well to SHOW people what you are doing
http://forums.adobe.com/thread/592070?tstart=30 for screen shot instructions
Free programs to get file information for PC/Mac http://mediaarea.net/en/MediaInfo/Download
Please detail in your reply to SG whether you are dealing with a codec issue related to a file that you are trying to import into Premiere Elements project or whether the issue is with the playback of any MPEG2 file in one of your Windows 8.1 64 bit players, including Windows Media Center? There is a consideration regarding video with MPEG2 video compression.