Blocking Orkut,gtalk & Yahoo

Type %systemroot%\system32\drivers\etc at the RUN dialog box.
There you'll find a file named hosts. Double click it and select notepad.


The file would end in something like

# For example:
#
# 102.54.94.97 rhino.acme.com # source server
# 38.25.63.10 x.acme.com # x client host

127.0.0.1 localhost

Just edit it so that it looks like:

# For example:
#
# 102.54.94.97 rhino.acme.com # source server
# 38.25.63.10 x.acme.com # x client host

127.0.0.1 localhost
127.0.0.1 www.orkut.com
127.0.0.1 www.yahoo.com
127.0.0.1 talk.google.com

Regarding firewall.
If you have a firewall then simply block port number 5222.
Its the port number used by gTalk.

In case you don't have a firewall.
Get one. And no, inbuilt windows firewall doesn't count here.