I'm hoping to harness the collective tips of superuser to gather recommended apps/configurations to keep a new desktop clean, virus free, and hopefully lower software rot.

I ask because I've recently come across tools like dropbox, deepfreeze, returnil, etc, and I'm curious what other ones are out there to protect a new box.

I personally am interested in Windows, but feel free to comment on whatever OS you'd like, freeware or otherwise. Ideally specify the OS in your answer(s).

One answer per program please. Then, rather than duplicate posts, vote for the program if it is already listed.

UPDATE:

It's been noted that there are other questions similar to this one [1], so I'd ask that these answers focus on security and protection.

[1] Related questions:

• http://superuser.com/questions/1241/what-are-some-must-have-windows-programs
• http://superuser.com/questions/1191/what-are-some-must-have-mac-os-x-programs
• http://superuser.com/questions/1430/must-have-linux-software
• http://superuser.com/questions/3855/must-have-networking-security-tools