Go to download.com and download the free versions of the following programs:
Spybot Search and Destroy
AVG Free Anti Virus
for extra protection you can also download the free versions of Ad-aware and Super-Anti-Spyware from download.com
AVG Free is a free anti-virus program, you can and should run only one form of anti-virus and this one is entirely free and very good value for freeware. The rest of these free programs are anti-malware scanners (some may give you the option to buy them, but they all have free versions in the installation instructions) which you can run as many of as you'd like, some will pick up on what others don't. Malware Bytes is my top-pick on this, but make sure to install at least two of the anti-malware programs I've mentioned.
Make sure your windows firewall is enabled. You can check this by going to your control panel (found in the start menu) and going to your windows security tab.
Otherwise, play it safe! Run scans regularly, stay away from sketchy sites, always scan your downloads, etc.
Best of luck.
Computer Science Security Student