That depends on the browser. IE will store its cache in "%HOMEPATH%\Local Settings\Temporary Internet Files", while Firefox stores it in "%HOMEPATH%\Local Settings\Application Data\Mozilla\Firefox\Profiles\user\Cache" where "user" is the current FF user(probably ending in .default). If you don't use IE or FF, you'll have to google where your browser stores its cache. I'm only really familiar with IE and FF.
If you want to clear all your browser's cache, you can do this in FF by going to Tools>Clear Private Data. In IE, you need to go to Tools>Internet Options and there should be delete options for cookies/history/temporary files somewhere in the General tab. Exactly where they are depends on if you're using IE6 or IE7, but they should be noticeable regardless.