It has been observed in some cases ,that while performing Repair on Dell systems, the keyboard gets stuck on Welcome to Setup
In order to resolved this issue, Press F2 to go to BIOS and make CD/DVD as the first boot device and then try to perform Repair installation.
**** Do not use dell quick boot ( F12), otherwise the keyboard won’t work****
Just wanted to add one thing : There is an option in the System Setup which is FAST BOOT or Boot USB Emulation. Turning it off would make the keyboard work .