_wmplayer.dll is associated with a BHO (browser helper object) that runs on Microsoft Windows Internet Explorer. It is not a core Windows file.
Browser helper objects are often used in spyware and adware programs. BHOs attach themselves to Explorer. This keeps them from being detected by Windows Firewall, which counts the object as a part of the browser.
_wmplayer.dll by itself is not necessarily a problem. However, some adware or spyware programs may create this file or disguise one of their own files as _wmplayer.dll. This may be located in the core c: \windows or c: \windows\system32 folder. The file has also been known to serve as a back door into systems, as well as a Trojan horse downloader.
_wmplayer.dll is know to record data input, which means that it can be a potential security violation if used by adware or spyware programs. A free scan with ZookaWare PC Cleaner can determine whether or not this file is valid or not.