Hello, and welcome to JustAnswer! I will make every effort to assist you. If you have any questions after reading my posts feel free to ask me!
There are lots of different possibilities.
Your port could be bad.
The keyboard could have a stuck key.
Start with the simple solutions.
Unplug the keyboard.
Then press each key firmly and make sure it comes back up (that it isn't sticky).
Plug it into a different port (if it's a USB keyboard, the same port if it is PS/2)
Start your computer.
Give it a full two minutes after coming back up to the login screen.
Let me know the results.
In the mean time I will switch this to a Q&A format question so you can come back to it at any time. I will also be notified when you make a posting.