only noticed this when i switched back to Windows 7 [64bit] from XP, if it says you dont have the right version installed [even after reinstalling] try the link below
http://webplayer.unity3d.com/download_webplayer-3.x/UnityWebPlayer.cab
Open in with winzip/winrar or similar then go the the folder
build|Windows|WebPlayerInstaller
& Run UnityWebPlayerFull.exe
Installer will run, no reboot required [didnt even need to close browser] and all is working again [Should work for all versions on Windows]