• Malwarebytes Antimalware: www.malwarebytes.org
• CCleaner: www.piriform.com/ccleaner
• Glary Utilities: www.glarysoft.com
Utilize a link checker: Many antivirus programs come with a link checker that will show a status icon next to links in search results and other webpages indicating the legitimize of the website the link is pointing to. For instance, they usually show a green icon when the website appears to be okay and then a red icon if there's evidence of viruses or other foul play. If it appears you don't already have a link checker, consider downloading the one from Web of Trust (WOT) at www.mywot.com.
Opt-out of installing junk with other programs: Many free games, programs and even updates by default will install junk along with them. However, most times you can opt-out of installing these extra things. Look for a Decline button or checkbox to uncheck when prompted to install the other things.
Use a sandbox to download risky programs: If you're a more advanced user, consider using a sandbox to download and test programs before fully installing them. Programs like Sandboxie (www.sandboxie.com) allow you to download and run programs in a virtually isolated environment, so even if it's a virus, there won't be any permanent damage.
Ensure your computer isn't already infected: If your computer already has viruses, malware or adware, you're more susceptible to downloading additional bad things. If your homepage or default search engine has changed or you're getting lots of pop-ups or errors, your computer may already be infected.
As always, ensure you have a good antivirus and extra antimalware program installed. If you think your computer is infected, I recommend calling a computer professional like myself, especially before you try to download anything.