Have you applied any passive protection on your computer? If not, you should do so as soon as possible. It's one of the most important things I look at when setting up a clients PC. While there is real-time protection like Norton Antivirus (which bogs down the computer by using up resources such as CPU power and memory), there is also passive protection which does not impact you PC's performance. It plugs in the security holes and tells the computer not to do various things which are guaranteed to give you a virus. I am going to list 3 different types of software I recommend which are all free. I hate spending money, and I'm sure you do too. If there's a free alternative, you bet I'm all over it.
The first is Spybot Search and Destroy. You can download it here on CNET. After installing and grabbing the updates, hit the immunize tab and then hit the immunize button on the top. This will help protect your computer from installing future spyware from websites and connecting to malicious websites. Here's a screenshot of what it normally looks like.
The second is Spyware Blaster. You can download it here on CNET. After installing and grabbing the updates, go ahead and hit "Enable All Protection" under the Protection Status tab. This will tell whatever browsers you have installed which sites not to go to. Here's a screenshot of what it should look like after you applied protection:
Lastly, there is Advanced SystemCare. You can download it here on CNET. While there are a lot of great optimizations this program can do, I will stick with it's passive protection features. Once installed, click the diagnose system button and have the program scan your machine for Security Defense. Once scanned, have the program apply the necessary fixes. This will help prevent spyware from being installed. I encourage you to look at the other things within this program as it is one of the easiest and most functional tools I've come across.
By doing these steps, you have highly increased the security on your PC without sacrificing computer performance. In fact, it isn't uncommon to notice a small increase of performance after applying these settings. If you have any questions, or some suggestions for future topics, leave a comment!