As Others May Have Said, There are a LOT of Reasons
First of all, if you just bought your computer at a store and brought it home, by now it must be obsolete so go buy another one (just kidding). First, make sure your PC is really clean. No, not just Virus free, you need to make sure your PC is checked for any malware, especially spyware. Make sure the software (Norton) is doing a good job getting rid of all that stuff. On the other hand, while there are many improvements in A/V software to go easier on performance hits. There are still some things to consider.
Most software that starts running when you boot up your PC does the following:
1) Computer boots
2) Software starts
3) Software goes out to the internet to check for updates
4) Software downloads updates
5) Software Installs updates
6) When updates get installed, software starts some kind of scan.
All of the above happens during the boot process. Now, consider what happens when you have a LOT of software (including Windows) doing all of this.
As someone else suggested (Doh_1), try to become a friend of MSCONFIG. The way I use it is to just allow WINDOWS services to start. Anything else that you see in MSCONFIG that is NOT Windows, temporarily tell it not to start that. Record your startup time. Then add one thing back at a time checking your startup times. Avoid stuff that downloads patches or updates until last. Why? Because, once it downloads, it may not need to download again for hours. Eventually, you will see what programs delay the completion of startup the most. Unfortunately, this is not an exact science as sometimes the problem is the "mix" of programs starting at the same time. At least this way, you will know what slows things down.
AntiMalware programs. Sometimes, in the rush to be as SAFE as possible, we wind up not just being 100% safe, but 200%, or 300%. If you are 100% or close to it, you need to ask yourself if you are going overboard. Three a/v programs is a bit much. Stick to a suite that only does ONE download to update all of its components and make sure you know when that download happens so you are aware of it and expect it. It's only recently that Microsoft allows DELAYED automatic starts. Not everything has to happen the very second that the power comes on.. Also, make sure you are not scanning high-use files over and over. Some of the new Symantec stuff only scans when it suspects a file has changed and not every time you read it.
Next, try using the task manager. If your computer is slow, run the taskmanager, usually by right-clicking on the taskbar and clicking on task manager. You want the "processes" tab. If you click on the CPU column once or twice, you can sort the processes by the percentage of CPU they are using. Have the higher processes near the top and, if you don't know what they are, look them up.
After all this, remember that there is still ongoing maintenance. Even with modern disk drives, a disk defragmentation session now and then can show some improvement as you will but Microsoft clusters adjacent to one another. Chkdsk can also help.
Finally, if you still can't find out what is going wrong, find a techno-wiz to help you. There is nother better than a second pair of eyes.