I had a similar problem on my Windows XP SP3. There would be 100% CPU usage upon Windows start-up.

Here is what you can try:

Workaround 1. Proceed to Avast → Settings → Update. Scroll down to Details. Check mark “My Computer is permanently connected to the internet.” Click OK at the bottom. Re-boot Windows, and verify if you still have the 100% CPU problem.

Workaround 2. If the above does not work, proceed to Avast → Settings → Update. From under Virus Definitions, click on the drop down box Settings. Select “Ask when an update is available.” Click OK at the bottom. This will switch the virus definition update from automated to manual. This is not an ideal solution, but it should mitigate your 100% CPU problem.