Good morning

I have a bizarre problem with my avast antivirus free edition 9 on widows xp ( cpu palomino computer ).
I use this pc every day. I go on the web in secure site. Every day at 11:30 a.m. I see that avast uses most of the power of the pc for the update of the virus definition. On the task manager appears correctly the instupdate.exe process. I don't see any error message during this operation.
Suddenly a popup of the security center of Windows tells me that the antivirus is not updated. I check on avast and I see this strange situation. I have the current virus definition installed but this definition was released few days ago. What can I do ? is my pc safe?
Thak you ^_^