Servlet Not Working At all

i m using tomcat4 at win2k.

i m unable to run my any servlet at all . Servlet Placed at root folder or with another context. I have checked invoker servlet into web.xml and uncommented the servlet/* from servlet-mapping tag. But it's n't work at all. Yet html pages under each context is working properly. i.e. index.htm etc.

i have accessed page my http://localhost:8080/manger and entered username and password. then i have created new host and then deleted.
Is it happening due to my this work ???
Please sugges me what to do ???
now if m trying to access page http://localhost:8080/manger with providing actual username and password it gives me error that
" FAIL - Unknown command "

I just started working with servlets today, I tried the first example at "chapter 1" at Hanumant Deshmukh's book (SCWCD Exam Study Kit), I made everything as discribed in the book, but it keeps giving me an error of "resource not found".
I use TomCat 4.1 as suggested by the book auther.