I use Ad-Aware, Spybot Search & Destroy and Spyware Blaster to keep my machine from getting all bogged down with spyware, malware and hijacks. These three are free applications without any spyware of their own. They either ask for donations or that you upgrade to more powerful stuff they have. I have used these programs for years with success. Google them to download. Do your own research and study on it.
This assumes you run an anti-virus program, and do your own clean-up of old files as well as the occasional de-frag disk thing. Scan disk and Windows integrety scan too. I use Symantec for that.
And don't forget to check for Microsoft updates, especially the critical ones.
Never download a file from an unknown source, even if they claim to love you, lol.
Oh, and rule number one. Never have anything on your harddisk worth stealing. Think about it.