There are a couple of ways you can clear the NPCScan cache. One way is by manually deleting the folder and the other is making a .bat file so that it will clear the cache then open up wow!
The cache folder you need to delete is located (generically) at:
C:\World of Warcraft\Cache\WDB\enUS\creaturecache.wdb
To make a .bat file open up notepad and copy the script below and save the file as Wow.bat (or whatever you wish to name it, and save it where it would be most convenient such as the desktop):
REM The next line deletes the creature cache
DEL "C:\World of Warcraft\Cache\WDB\enUS\creaturecache.wdb"
REM The next line will start World of Warcraft
CALL "C:\World of Warcraft\World of Warcraft Launcher.exe"