I have installed avast on my home pc, i restarted the pc and it started scanning . When the scan starts the keyboard stops responding. Even worst it wont let me log on in safe mode either … WHAT CAN I DO ??
I found this post that might answer your question
http://forum.avast.com/index.php?board=2;action=display;threadid=111
Also try pressing and holding f8 as the computer is loading up. You should be able to get to the safe mode. However, uninstalling Avast has proven to be a challenge for some, I found the easist way is to double click the installer which will ask you if you want to remove Avast once it sees that it is already installed
Hope the thread above helps
James
Do you use an usb keyboard? You will have trouble with it because drivers are not loaded at that time… 
See http://forum.avast.com/index.php?board=2;action=display;threadid=4468
Hello,
I have installed avast on my home pc, i restarted the pc and it started scanning . When the scan starts the keyboard stops responding. Even worst it wont let me log on in safe mode either ... WHAT CAN I DO ??I have the same problem and now i know why :
Do you use an usb keyboard? You will have trouble with it because drivers are not loaded at that time...
But now how do I logon to winXp? Could you tell me how to disable the boot scan? ???
Thx
Toxfox
If you press F8 when starting Windows and select ‘Last known good configuration’ from the boot menu, the boot scan should not be started.
Thx!!!
Just an after-thought – this was news from my son recently. He says that most relatively new/recent computers should have a “quick-and-dirty” set of USB drivers built right into the BIOS, so you should be able to use a USB keyboard even at the POST level.