my desktop refresh two times .

Avast Free Antivirus / Premium Security
1

my desktop refresh two times ,when i my computer starts up.

only use avast free.there is no other security software .

2

Hello sjm666cn, welcome to the forums.

First, try to repair avast installation.
You can do this from Control-panel → Add/Remove → avast.

What is your OS (XP,Vista,7…) and avast’s version (latest is 5.0.545)?
If you disable avast, does the problem go away?
If so, try to enable Settings → Troubleshooting → “Load avast! services only after loading other system services”.

Regards