No Internet connection is 100% secure - regardless of which Internet provider is used. Put simply, some connections are "less insecure" than others.

Most security principles which apply to dial-up modem connections also apply to ADSL. However, because ADSL is an "always-on" connection, there is increased likelihood that an unauthorised attempt might be made to access a computer or network.

A number of precautions are essential, for example:

Turn off "file and printer sharing" on all computers on your network. If you leave sharing enabled, be aware of the increased potential for unauthorised access to and/or manipulation of your data.

Install anti-virus software on all computers. Ensure that the software is regularly updated.

Conduct regular backups of your data - especially those files which cannot be restored from CD-ROM.

Archive your backup media (tapes, etc) securely in an off-site location. Ensure that you retain backup media over a reasonable number of months; it may take time to detect an intruder.

Be careful about opening e-mail (especially attachments), particulary when you do not recognise the sender. E-mail is a popular method of distributing viruses and trojans.

Regularly check for security/critical updates to your operating system (Windows, MacOS as applicable).

If you are a business or organisation, make security a key management issue.

There are many factors to consider - router firewalls, password policies, user behaviour, and many more.