Be sure the programs you d/l are coming, if at all possible, directly from the source. Anytime you use a host site, such as Cnet, you risk the site adding unwanted goodies. This article shows a good example of what happens when you d/l from such a site.
Heres What Happens When You Install the Top 10 Download.com Apps
Malwarebytes (the free version) can help keep your system clear. Be sure to uncheck the "start free trial" box after installation is finished. If you wish to opt for the pro version, that features active background scanning & automatic updates.
Be aware of where you go online & that visiting questionable websites can result in drive by downloads, that will infect your system without you knowledge.
You may wish to check if your system is infected with a rootkit. TDSSKiller is a good tool to check for this.
When running TDSSKiller, launch the program, click on the blue text "Change Parameters" & check the box marked "Detect TDLFS File system." Click OK & then run the scan.