Welcome to the forum!
This prompt is asking you to log on to your FactoryTalk directory, which all of your Rockwell products that have security enabled use.
By default when you install version 20 of RSLogix 5000, security verification is enabled, but the "network" security server is the local host PC.
Question: Are you using FactoryTalk AssetCentre software? If so, then most likely you are set up to use a remote server PC as your security authority. In which case, you should have already had a user ID and password set up to login to the server.
If you are not using a remote server for security, then this login prompt is just asking for your user ID and pw for the local windows login credentials.
To get rid of this prompt, go to Start - All Programs - Rockwell Software - FactoryTalk Administration Console. Log in to "Local" as your windows user (needs to have administration privileges), then go to Policies - System Policies - Security Policy and enable "Single Sign-On". What this does is once you log on to windows, then you are globally logged on to the FT directory, and you won't have to log into your products separately.
Now, if this doesn't work, if it doesn't take your windows user creds, then either you don't have administration right to your pc, or your directory has gotten hosed, and is pointing to a remote server that doesn't exist. I have seen this happen on a guys PC who tried to open an .ACD file that had remote security enabled. It basically told his RSLogix that he needed to login to our remote directory, which he could not do.
To fix this, we had to reconfigure his FactoryTalk NETWORK directory (using the wizard) to use the local host as the network server to get his Logix working again (sounds worse than it is).
If you need to go this route, post back, or PM me and I can walk you through the procedure. Unfortunately, I've learned WAY more about FactoryTalk security in the last few months than I ever cared to!