Seriously the first thing is to install decent anti virus, firewall and malware programs. Microsoft Essentail is good as is AVG amd Malwarebytes.
Make sure they are set to scan daily and have auto update switched on.
Run a program like ccleaner at least once a week.
Keep Windows Updated.
Most importantly is don't do anything stupid.
Don't open unknown attachments in emails, don;t visit dodgy websites, don't use P2P sharing applications like Limewire.