I'm attempting to fix my wife's PC, an Athlon XP 1800. Last Tuesday, it would not boot Windows, stating that \windows\config\system was missing or corrupt. Attempting to restore it did nothing. I copied her data - 9GB worth - to another computer, formatted the drive, reinstalled XP and SP1. Then I installed the applicaitons that were on before (Office 2000, Mozilla, RealPlayer, iTunes, MSN Messenger). Got everything working. All this time the floppy drive was not connected and it was set to not present in the CMOS.
I get the machine home and connect the floppy drive. After enabling the drive in CMOS, I try to start XP. I get a blue screen stating that an invalid config was present. I then disconnect the floppy and disable it in CMOS and try again. It boots into XP. Great! I shut it down, reconnect and reenable the floppy drive and restart it. It would not boot Windows, stating the \windows\config\system was missing or corrupt.
Of course, after seeing that message again I really wanted to throw the computer out of the window, but my wife would kill me if I did that. 
Here are the contents of the machine:
Athlon XP 1800+
384MB PC133 RAM (1-256MB, 1-128MB)
40GB WD Caviar hard disk
3.5" floppy
OptoRite 48x24x48 CD-RW
ECS K7SEM motherboard (SIS 630/730 chipset, built-in video/sound/network)
Generic 56k PCI Winmodem
USB internal card reader (connects to USB header on motherboard)
She doesn't do 3D gaming, so the built-in video is sufficient for her use. Primary uses are email, downlaoding video files and web browsing.
Any suggestions?
Thanks ahead of time!
Brian
Is the mobo BIOS at its most current version? I'd check to see if a flash upgrade is available.
Hey ,I know that problem.Did this installation worked before, because three years ago every time when a friend of mine tried to install it on his computer it didn't wanted to start and was giving the mising SYSTEM file error (white fonts on black screen).He tried to install it on one of mine computers (Mitsubishi Apricot 550 266Mhz PI 96Mb Ram, would you belive
and it installed fine.He tried again on his with no result.I think he find another installation CD of XP ,and it installed fine.
1. Check the setting for Plug and Pray operating system in the BIOS settings. Change the boot order settings so that the floppy drive isn't accessed during POST or Windows startup.
2. Cut your losses. Disable the internal floppy drive and treat your wife to an external USB one or just use a USB flash storage device.
Phil
Shoot, I just fixed a problem like that on one of my wife's frien'd systems.
The fix was to restore the most recent registry backup to the system file.
If you google around a bit for the error message, you should be able to find a good writeup of what to do. (IIRC, there was a good one on ExpertsExchange.)
--John
I ended up enabling the floppy drive, then doing a reinstall of XP on it. So far so good. The applications are still there, but I will double-check to see that they still run.
SP1 is installed and I'm installing the updates via NeoWin.net's AutoUpdate XP program. It allows MS's updates to install without connecting to the internet. Takes about 2.5 hours to run, but when it is done there may be one or two updates left to install from the Windows Update site.
It turned out to be an incompatible DIMM. The 128MB DIMM caused a trap error 0x00000074. I tried booting the machine with only the 128MB and only the 256MB DIMM. When the 128MB DIMM was in, the same error occured.
It seems to work OK now.... so I'm leaving it alone.
Brian