Help Windows XP Professional

[montani]

Yesterday evening
while the PC connected to internet
appared a little window on right-low of the screen with the label
Antivirus have download the last version of the database
Please click for information
After the click Eudora breezed.
I try to launch avast update but
After the hard off the PC doesnot restart
and the file
\WINDOWS\SYSTEM32\CONFIG\SYSTEM corrupted

I tryed to copy SYSTEM.SAV to SYSTEM after the DOS restart
but at the start ask the CD for installation.
How is possible to recover the original file system?

Sunday I have download Avast 4 and i have scan approfonditely the hard disk.
I have Windows XP Professional, Eudora, Avast4, IE 6
Thank you in advance for your reply.

[Pavel]

Yesterday evening while the PC connected to internet appared a little window on right-low of the screen with the label Antivirus have download the last version of the database
Please click for information After the click Eudora breezed.
I try to launch avast update but  After the hard off the PC doesnot restart and the file \WINDOWS\SYSTEM32\CONFIG\SYSTEM corrupted

This does not look as avast problem to me - the database update is just the data file transfer and it does not have anything to do with either Eudora nor the SYSTEM file at all...