Start-up

There is a very slim chance that it's hardware but to confirm, there are Dell diagnostics on the hard drive accessed by F12 at initial power on boot up.  You can run a complete diagnostic. 

Slow Windows startup can be malware/spyware, startup programs, a fragmented hard drive (have you ever defragmented the hard drive?), or a Windows issue.

The first thing to do is defrag the hard drive.  Go to My Computer (or Computer depending on your OS) and then RIGHT click on the C drive then LEFT click on Properties then click the Tools tab.  Run the Defragmentation. 

Second, if the defrag didn't help, run a complete malware/spyware scan with an up to date malware program and malware data file (you can download the free MalwareBytes program if you don't have one   Click Me )

Third, make sure Norton's antivirus program and antivirus data files are up to date (If you never paid for Norton past the initial trial period it is out of date and of no use).  If you do not have a paid valid Norton subscription, you can download the free Microsoft Security Essentials, uninstall Norton and install the Microsoft and you will have a current and up to date antivirus program.  Click Here for MSE

Try the suggested steps and see if that helps.  If not, a complete reinstall is probably in store.  If that's the case you need to backup your user data before doing the reinstall.  There is a Dell "PC Restore" on the hard drive that will reinstall everything back to the original factory condition.  HERE is the Dell installation instructions.