how to restrict web sites?

dear all

in our company i want to restrict web sites for the users. they should access only allowed sites. how should i do?

i enabled content advisor in IE, enabled passwords, i enabled the ratings also. and i gave the allowed website names.

but some machines browsing othersites beyond my restrictions. some sites asking password for official site also.

i want the following setup.

1. users should browse only allowed website without passoword.

2.users should not be allowed forother sites.

how can i restrict. please help me!

regards
sakthi