Please list your Dell model for better suggestions.
First, you need to backup any personal files to an external drive ASAP as your hard drive is starting to fail, hence the slow start times. Once the drive fails, you will be unable to retrieve your data without the high cost of data recovery services.
Replace the hard drive with a SSD for faster speeds. They cost a little more, but are worth the extra price. From there, you have two options: Attempt to clone the existing hard drive to the new one, or reinstall Windows and your programs on the replacement drive. Cloning requires some additional equipment in order to access both drives at the same time, as well as a specialized program to correctly transfer all data and partitions. Reinstalling Windows either requires the original system recovery media provided by Dell, or the Windows 10 installation media, assuming that Windows 10 is already installed and properly activated on your computer.