If I had to choose the one security product that I could not do without, it would have to be Sandboxie www.sandboxie.com
There is a free version and there is a paid version. The free version provides almost the same level of protection as the paid version but has some limitations. The paid version gives a lifetime of free updates. Unfortunately, it works only on Windows.
As the name suggests, it sandboxes programs, meaning that they run in their own "virtual" environment and do not alter the host operating system (unless you choose to do so).
Uses for Sandboxie are many but include:
1 Running your favorite browser in a sandbox. Any nasties that you might accidentally download to your machine, eg "drive-by" malware, will stay inside the sandbox and not infect your machine. When you close the browser, everything from the browsing session can be deleted.
2 Installing software applications into a "permanent" sandbox. Many applications want to install toolbars, change your home page, change your favorite search engine, add spyware and a host of other undesirable things. And even if they don't do this, they still can pollute your registry and system32 folder with unwanted garbage (dll bloat). Installing the app into its own sandbox can stop this from happening. Unfortunately, not all apps can be installed into a sandbox, eg those that install their own drivers and/or services, but a good many can.
3. Running things that you're not sure about. For example, that pdf file you downloaded might have something bad in it. Run your PDF reader in a sandbox and you'll be protected.
4 On top of this, you have Buster's Sandbox Analyzer (BSA), a free tool written by a Sandboxie enthusiast, which allows you to run a piece of software and analyze what it does to your system (eg registry entries, files added/modified/deleted) without any of this actually happening to your real OS.
Sandboxie is well worth a look.