Source: Photoshop Support Knowledgebase

Issue
When you start Adobe Help Center, the application appears in the taskbar but the Help window doesn’t appear on the screen.

Source: Photoshop Support Knowledgebase

Issue
When you use the brush tool in Adobe Photoshop CS2, your brush becomes unresponsive and Photoshop freezes.

Error: “…serial number is missing or invalid…” occurs when you start Photoshop (CS2 on Windows)

Source: Photoshop Support Knowledgebase

Issue
When you start Adobe Photoshop, it returns the error, “Your Adobe Photoshop user name, organization, or serial number is missing or invalid. The application cannot continue and must now exit.”

Source: Photoshop Support Knowledgebase UPDATED

What’s covered
How Photoshop allocates RAM
New capabilities with 64-bit processors
Virtual memory and the Photoshop scratch disk
Determining how much scratch disk you need
Out of memory errors
Maximum file, hard disk, and scratch disk size
Adjusted Refresh plug-in
Bigger Tiles plug-in
This document describes how Adobe Photoshop CS2 uses RAM and scratch disk space.