Particularly if it asks for more than an activation key. like a credit card number. If so it's malware and needs to be removed. Often times it changes your folder/file permissions for access forcing you to deal with this and not just be an annoyance. yes your screen can go black (if permissions to your wallpaper are changed)

A normal re/activation screen generally occurs when you have made too many major changes to your system like changing the motherboard, hard drive, videocard at the same time. The background changes to black and you can't change it back. You retain most of your functionality and access to your files while you deal with this. Bios/mbr corruption can cause this too.

If you bought the system second hand, it's likely it did not get properly activated with a genuine key.