Look for an option in your browser's menu (under Tools, Settings, Preferences, or a similar option) to clear out temporary internet files/cache. Different browsers have different ways to do it and sometimes refer to it differently.

For IE on WinXP, click "Tools" then "Internet Options" then (under "Temporary Internet Files") click "Delete Files"

For Safari on OSX, click "Safari" then "Empty Cache"

For other browsers, you should refer to its Help files.