After install of AVG 2012 Internet Security on my XP SP3 Pro , all was O.K , made full scan, updated the program, but next day can't start PC receiving a message "fastfat sys. file" is corrupted.
Try in Safe Mode , but with the same result.
Try also by inserting "CD" to fix system- "r"- but at the end of importing files and drivers , before screen with the available options appears on , the same message was shown telling  me I have to restart PC as a/m file is corrupted and Windows can't start.
Please advise me what I can do- if any – to fix this problem.
 

Have you tried running system file check to identify and replace corrupt system files?

Start > Run sfc /scannow

You need to have your original OS CD mounted.

sam

 I can't start Windows in any mode.
 During start up , after short period of load, PC stop, advising me to insert "CD" and press "r" to repair - I repeat try also this , but after load of numbers of files process stop loading shoving me the same message " file fastfat sys. is corrupted" and- Windows couldn't start and I should restart PC.
This cycles is turning again and again.
 I am not sure what can be the reason why this occurred, is it software or hardware problem .
By the way, this file is a part of System 32\Drivers.
This is the firs time happen to me that I can't start Windows - so it could be bad sector in HD , some bad connection of certain hardware elements , or.....   

From what I've read on the net, this can be caused by a bad memory stick. What memory have you got in the machine? If you've got two sticks, try booting with each one on it's own to see if this solves it.

@MarcLodge,
Thanks for advice.
I  have (3) RAM on the slots - two of 128 MB and one of 256 MB- totally 512 MB- probably there lies the problem.
Few days ago I took my PS to one Service to clean it from the dust
and if possible to change my processor and RAM wit bigger one.
Unfortunately, as my PC is old machine, they were unable to improve any of this and after several attempts, returned my old components.
Probably something goes wrong with RAM insertion, so I'll go next days to ask them for fix this problem.

can you boot in Safe Mode?

Thanks guys for help, problem sorted.
All was caused from bed contacts of RAM- cleaned contacts and everything start to function fine.
V