Please post the exact model number of your system. How long does it take to boot? Do you have an HDD or SSD in the system? How old is the system? Did the problem start all of a sudden? Was the system working normally or has the problem been getting worse? After the system boots is operation fine or does it run programs slowly as well?

A slow Microsoft Windows boot up can be caused by many different issues. The following document contains various suggestions and tips that can help improve the overall speed of your computer's startup. It should be noted that this page only covers a slow computer boot time; not what to do if your computer is running slow overall.

Disable startup programs
Programs that are loaded as the computer starts up remain active in memory. Consequently, they are one of the main causes of a slow boot into Windows. Disabling programs you don't often use from automatically loading up each time the computer powers on can decrease boot time.

ScanDisk and Defrag
Using Microsoft ScanDisk and Defrag or similar disk utilities can help verify your hard drive is running free of errors while organizing stored data in the most efficient manner possible. We suggest running both of these utilities at least once every few months.

Insufficient hard drive space
For your computer to run optimally, it needs at least 250 MB of available hard drive space. If you have less that this amount, slowdown in both general performance and the boot time can occur.

Update drivers and Windows
Corrupt, incorrect, or out-of-date drivers can cause many different issues. Make sure your computer has the latest drivers as well as any available Microsoft Windows updates.

Registry cleaner
Although we do not often recommend registry cleaners, in some rare situations they can help improve the overall performance of Windows including startup. If you have run through all the previous recommendations without strong results, you may want to clean your system registry.

Format and reinstall
Although you may need to backup some of your files and the process itself may be a bit time-consuming, you may want to erase everything from the computer and reinstall Windows. This process makes your computer run like it did when it was new.

Hardware upgrades
We saved these options for last because they have a   cost to the user associated with them. That being said; they are some of the most effective ways to increase performance and decrease boot times.

Adding random access memory
Installing additional memory (RAM) to the computer helps increase the overall speed of the computer and in some cases can decrease the boot up times. If your computer is running less than two Gigabytes of RAM, it may be time to upgrade.

Adding a solid-state drive
An SSD is the most effective way to raise performance across the board for any computer. On both desktop and laptop computers, moving Windows from a standard hard drive to a solid-state drive will dramatically lower your boot times. Amazon.com has a full array of SSDs.

Also try to follow these, which might help other users who stumbles upon this

Disable Visual Effects

To Disable Visual Effects follow these steps:

1. Click Start
2. Search for "Control Panel" and Select it
3. Click on "System"
4. Click on "Advanced System settings"
5. Select "Advanced" Tab
6. Click "Settings" from Performance section
7. Select "Adjust for best performance"

Disable Windows Tips

To turn off these tips, go to Start -> Settings -> System -> Notifications & actions and toggle off Get tips, tricks and suggestions as you use Windows.

Source: 10+ Ways to Decrease Windows Boot Time

What is the exact system model? What is the current OS installed on the system?

When did the issue start? Is there any damage on the system?

Update the BIOS - http://dell.to/1n3X2po  - enter the service tag and download the BIOS, update and restart the machine and check.

Restart the system and press f12 key on startup - choose diagnostics. Let it run the diags and if you encounter any errors - make a note of the complete error and report the same to us. These are onboard diagnostics and they might possible pick some error if the hardware is not functional. Also, run an extended / thorough tests on the HDD and memory to check for any possible errors. 

Restart the system and check in safe mode - http://bit.ly/1LHcO2c

Enable the built in admin account and check - http://bit.ly/1JoFhcw - This will help isolate if the issue is profile related or not.