Hello and Welcome to PCHF

First off, you do not need to run SFC after a fresh installation, why you would is beyond me.

Windows XP has the ability to protect itself from system instability caused by 3rd party software overwriting important system files. This used to be (and still is in fact), a problem with Windows 95 and Windows 98.
With the introduction of Windows Millennium Edition, Microsoft made a real effort to stop this from happening.

By default, Windows File Protection is always enabled and allows Windows
digitally signed files to replace existing files safely. Currently, signed files are distributed through:

#Windows Service Packs
# Hotfix distributions
# Operating system upgrades
# Windows Update
# Windows &nbspevice Manager

If you introduce a file replacement in any other way, Windows File protection will overwrite your file!
An important part of Windows File Protection is the command line utility:
System File Checker (sfc.exe)
You will often see references to scannow sfc in online newsgroups etc. This is a great tool for troubleshooting Windows XP problems.

I do not need to, but I want to run it. The fact remains that it FAILS. I inquired as to the cause of that failure, not the of personal views on wether one should-or wouldn't- run sfc /scannow.

So..anyone knows why it happens?

Hi there, as GeekGirl correctly pointed out, SFC, is used to check if Original System files are still intact (the files that Windows protects from being overwritten). If these are not in the correct locations or are missing or damaged, then when you try to run SFC, it will prompt you for a Windows CD, so it can put the original files back.

Does the Windows event viewer list any errors relating to this georgeks?

No errors on event viewer, nothing to indicate something is out of order-whatever that means in a Windows installation...
The message I get after sfc /scannow completes is:

Microsoft Windows [Version 6.0.6000]
Copyright (c) 2006 Microsoft Corporation. All rights reserved.
C:\Windows\system32>sfc /scannow
Beginning system scan. This process will take some time.
Beginning verification phase of system scan.
Verification 99% complete.Windows Resource Protection found corrupt files but wa
s unable to fix some of them.
Details are included in the CBS.Log windir\Logs\CBS\CBS.log. For example
C:\Windows\Logs\CBS\CBS.log
C:\Windows\system32>

sfc /scannow has failed on every system I managed to lay my hands on, 5 notebooks and 3 desktops. All -except 1 notebook-were new, all had OEM versions of Vista (Home Premium).

Just for the record: Extensive documentation about sfc /scannow exists, but is only for Windows XP, I do not mean that it is not applicable to Vista, just that there may be differences.
I know that I do not have to run sfc /scannow, I just did it and found it strange that it fails after installing updates -This I verified 4 times on my system, since the rest of the systems I checked were OEM versions and pre-installed when they failed.
One person that performed the check on her system for me also reported fail, that was on an Acer notebook.