It's even easier to make a batch file and toss it into your startup folder, open notepad and type in
Code:
DEL /s /q "D:\World of Warcraft\Cache\WDB\enUS\creaturecache.wdb"
DEL /s /q "D:\World of Warcraft\Cache\WDB\enUS\npccache.wdb"
(Replacing my paths with where you installed your wow to ofcourse)
Save the file as "Whatever.bat" wherever you want, then drag and drop the saved file into your Start Menu --> All Programs -- Startup, creating a shortcut there.
Now every time you start up your computer it cleans your cache and no need to worry about it anymore