Many reasons!
http://support.microsoft.com/default.aspx?scid=kb;[LN];830086
"Windows Update has encountered an error cannot display the requested page" error
message - Microsoft Windows XP Home Edition SP1
http://support.microsoft.com/?kbid=193385
WINUP: Problems When Viewing or Downloading From Windows Update
Windows XP, Windows 2000, Windows Millennium Edition, Windows 98 Second Edition &
Microsoft Windows 98
Windows Update Troubleshooter
http://v4.windowsupdate.microsoft.com/troubleshoot/
Windows Update has encountered an error and cannot display the requested page.
Windows 2000, Windows 98 and Windows 98 Second Edition, Windows Millennium Edition,
Windows XP
http://support.microsoft.com/default.aspx?scid=KB;EN-US;836962
You receive an "Error 0x800A138F" error message when you click a link on the Windows
Update Web site WinXP & Win2000
When you click Scan for Updates on the Windows Update Web site, and then click one of
the active links in the left pane, the Web site may not appear, and you may receive
the following error message:
Windows Update has encountered an error and cannot display the requested page.
Error 0x800A138F
The Update.log file may indicate that an error 0x800C00008 has occurred.
You receive Initialization error 0x80072F78 while the Windows Update Web site is
loading
http://support.microsoft.com/?kbid=836985
When you try to use Microsoft Internet Explorer to update the Microsoft Windows
operating system from the Windows Update Web site, you may receive an error message
that is similar to the following:
Windows Update has encountered an error and cannot display the requested page.
