XP Pro Display Properties Desktop

Question

On my XP when I wanted to change the wallpaper I got up Display Properties then clicked on the Desktop tab. I get an error message to send to Microsoft.Something has changed, I must now log in as a user, in the past I went right to my "normal" screen.Everything else seems to be normal. I run BLINK and did a virus scan, "No malware found".Any ideas? Thanks in advance.Ray