To save cost and possibly improve performance, I advise that you DO NOT install an antivirus scanner product that comes with a firewall. (I have specific recommendations for that in the software firewall section.) At this time I prefer Bitdefender for best protection from malware. Some may prefer another antivirus product (such as Avira) that might in some cases be more universally compatible with specific computer games that they run on their computer. You can get either one of these products here. BE SURE TO UPDATE THE VIRUS SIGNATURES BEFORE YOU USE IT TO SCAN YOUR COMPUTER.
You should un-install ALL your security software BEFORE installing your antivirus product. If you don't, there will likely be software conflicts. However, have your antivirus install file already downloaded (or use installation disks) and be ready to install your resident antivirus product immediately after you uninstall the other software.
IMPORTANT NOTE: If you have decided to use Ubuntu Linux to surf the Internet and for all social networking AND you only use Windows for running Windows programs (and downloading their updates) and for updating Windows itself, you may consider using a free antivirus product such as Avast for your Windows side of your computer. However, if you use Windows to surf the Internet, use any browser within Windows to access your e-mail, use social networking applications to communicate with friends, or use services like Facebook, Twitter, or Linked-in, then use a more powerful product with Windows, as recommended above.
After scanning your computer very completely with 1) your resident antivirus program and afterward with 2) SuperAntiSpyware, and 3) Emsisoft Anti-Malware, in that order, (with all the updated malware signatures for each), if your computer still does NOT run well, your computer registry may need to be repaired, or there may be other problems. Follow the specific directions listed here.