Here is the host file I deploy in my PC labs - my users have pretty much admin rights (not my decision) but the vast majority of them don't know about the hosts file. P2P traffic is blocked by the network guys, but this prevents them from downloading some of the stuff in the first place.

Using the hosts file is not bulletproof, as they can download it elsewhere or even edit the host file - it is just a way of slowing them down. The beauty is when they try and download "Instant messenger" of choice, it acts like the page is down, so they forget it about it and try again later, same thing.

The idea here is to block both the download site and the servers that are used to authenticate and login. This does not affect people checking their email on hotmail or AOL-webmail, but it makes using an IM much more difficult.

Some of these, like Weatherbug, are harmless, but eat up a huge amount of bandwidth, especially when it is installed on many machines.

# -------------------------

127.0.0.1 localhost

# Block AOL IM
127.0.0.1 login.oscar.aol.com
127.0.0.1 aim.com
127.0.0.1 www.aim.com

# Block MSN Messenger
127.0.0.1 gateway.messenger.hotmail.com
127.0.0.1 login.gateway.hotmail.com

# Block ICQ
127.0.0.1 login.icq.com
127.0.0.1 http.proxy.icq.com
127.0.0.1 icq.mirabilis.com
127.0.0.1 http.proxy.icq.com
127.0.0.1 login.icq.com previously

# Block Yahoo IM
127.0.0.1 msg.edit.yahoo.com
127.0.0.1 messenger.yahoo.com
127.0.0.1 http.pager.yahoo.com
127.0.0.1 scs.msg.yahoo.com
127.0.0.1 scsa.msg.yahoo.com
127.0.0.1 scsb.msg.yahoo.com
127.0.0.1 scsc.msg.yahoo.com
127.0.0.1 msg.sc5.yahoo.com
127.0.0.1 msg.yahoo.com

# Block Weatherbug
127.0.0.1 weatherbug.com
127.0.0.1 www.weatherbug.com

# Block P2P
127.0.0.1 kazaa.com
127.0.0.1 kazaalite.com
127.0.0.1 limewire.com
127.0.0.1 bearshare.com
127.0.0.1 edonkey2000.com
127.0.0.1 filedonkey.com
127.0.0.1 winmx.com
127.0.0.1 www.kazaa.com
127.0.0.1 www.kazaalite.com
127.0.0.1 www.limewire.com
127.0.0.1 www.bearshare.com
127.0.0.1 www.edonkey2000.com
127.0.0.1 www.filedonkey.com
127.0.0.1 www.winmx.com