I own Photoshop CS2. Your version may vary. But most of this is useful across versions.
In PS CS2, you can find most of this in the Help under:
Help Topics > Work area > Tools
✦ You bought and paid for your Photoshop CS2. Adobe likes money.
✦ Adobe shuts-down their registration server, 'cause they no longer sell people copies - they rent their stuff for a monthly fee (which from a an International software vendor point of view makes a lot of sense).
✦ You're happy with CS2, and don't want/need perpetually-up-to-date image editor, for a monthly fee.
✦ When you install your purchased, licensed software on your new PC, can't register because the license server no longer exists.
✦ Photoshop opens a "please register" nag screen every time you open CS2.
Go to the Application folder for CS2 and delete this file regsresen_US.dll.
That's it. Nag screen no more. Grog happy!