It is a common expectation that software should  be free these days. I know I enjoy being able to download and use programs for free. Lately I have been noticing that some free programs bundle in other software and unless you look carefully you will unwittingly get software you didn't ask for. An example is Avast Free Antivirus. When you go to install the package it will include Google Chrome browser by default. In itself this is not a problem but the more unnecessary software that is installed, the more your PC will slow down, so it is best to avoid it if possible. By paying attention to the options during the installation (select custom or advanced) you can generally choose  not to install the additional programs.

Then there are the more devious programs that will install adware onto your PC. Examples of this are the Babylon toolbar. This will redirect all your searches in your web browser to the babylon.com website.  It is difficult to remove this toolbar as the uninstaller doesn't remove the extensions from the browser. Similarly the searchnu/searchqu toolbars do the same thing. These adware packages (also known as Potentially Unwanted Programs or PUP) can hijack your browser and redirect traffic via their website. A good rule of thumb is to only download software from reputable websites or the vendor's website (such as microsoft.com or dell.com etc), especially drivers and system files. Of course I would strongly recommend against downloading cracked software (also known as warez, software that bypasses the license restrictions) which could have any kind of malware embedded. Even with antivirus software installed, these programs can infect your PC.

Something else to avoid installing are packages that claim to improve PC performance. More often than not these packages make things worse rather than better. There are utilities available that will cleanup your PC but they need to be used judiciously and require a certain amount of computing knowledge to be effective.

Lastly make sure you have your data safeguarded by backing it up to another disk/external hard drive. Windows backup (from Windows Vista and upwards) is quite effective and free. This will allow recovery of system files in case of infection. Another useful tip is to create a restore point before installation of a program. This will allow you to rollback system changes if you find that the software is malware or causes problems. You can find the restore point under System Restore in the Control Panel.