Most of these links are to, or lead you to free software.  I have used some but not all.
Look before downloading, some may have nonfree versions as well.

Some important things to have on a computer connected to the internet are Anti-virus software and a firewall.
Antivirus - software block and removes virus'
Firewall software blocks against other ways of computers trying to break in to your compter.
Anti spyware software is also very useful for blowing away a lot of those add programs that find their way on to your computer.