There are two kinds of popups - browser popups, and messenger spam.
For the former: try
www.panicware.com or the new Google toolbar
(
http://toolbar.google.com). Also might want to check out
www.lavasoftusa.com and download AdAware (free) - install it, launch it,
update it, run it. It will check for spyware/adware/scumware and browser
hijacking.
For the latter:
You need a firewall. 'Messenger spam', is not in itself harmful,
but is symptomatic of a larger problem - your computer has ports open from
the Internet.
You can disable the Messenger service, but that is unwise as a solution as
you'll still have the underlying problem that caused you to get the messages
in the first place - consider the messages a useful warning that you have no
protection from the Internet. You need a firewall. 'Messenger spam', is not
in itself harmful, but is symptomatic of a larger problem - your computer
has ports open from the Internet. This is very important at all times, but
especially now that everyone's gotten a major wakeup call due to the recent
worm virus activity...
For a standalone machine, see
www.sygate.com for a free personal firewall,
or if using Windows XP, you can enable the built-in internet connection
firewall (ICF).
See
http://securityadmin.info/faq.htm for more info.
