Avast anti virus disapperears after up date

My antivirus vanished after I updated it. Asked for a restart after update and when I restarted windows, Avast was no longer there. Tried talking to support but they were no help to me. I did get an error message saying a dll file was missing.
After getting no where with support tried using my own head. I deleted avast did a reinstall and ran a virus check. There was a virus.
So my problem is when avast is running everything slows right down. cpu is up to a 100 percent and when I go to task manager it say avast is using 95 percent of cpu.
Does this makes sense? I have defragged and did all the things suggested.
Any help will be really appreciated. I have a dell insperon 1525 4 years old but hardrive is 3 years old.
 
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