Mostly this is done by robots that try to log on to your user ID's. They
appear to try obvious passwords, such as "XXX", blank, "user", "admin" etc.
You can find out how often the logon attempts are occuring by going to
Control Panel, and select Administrative Tools, then display the Event Viewer
and select "Security Logs".
You may be able to fix this by installing a DSL router, but it's probably
better to get a proper firewall.