There are a few issues that I see.
It's almost always a bad idea to override the service method directly.
Instead override doGet or doPost.
Instead of writing directly to the servletOutputStream, use a printWriter by calling response.getWriter.
Your web.xml heading specifies
servlet spec 2.2.
Tomcat 5.5 supports servlet spec 2.4.
This might not be a big problem with this servlet but you are going to run into problems later if you try to learn any of the newer features (filters, listeners, EL, JSTL, etc..).
Lastly, and this is probably what's causing your 404 error, the url-pattern in your mapping specifies /hello but in your url you're requestiong /HelloWorldServlet
Change one or the other.