I'm afraid we'd need some more info to find out what's going on there.
If possible, please do the following:
1. Wait for the CPU usage to appear and give it some time (say one minute)
2. From an elevated prompt, execute the following command
sc.exe control "avast! Antivirus" 254
3. It will take a moment - and a dump of the AvastSvc.exe process will be created in
C:\ProgramData\Avast Software\Avast\log folder; it will be a file called unpXXXXXXX-manual.mdmp (where X are some numbers) and will probably have hundreds of megabytes in size.
4. Compress the dump (7-Zip, ZIP, RAR, ... whatever) and upload it under a unique name to
ftp://ftp.avast.com/incoming5. Post the name of the uploaded file here; I'll check if the code seems to be stuck in some weird place
Thanks.