High CPU Usage when local scan is active / PC gets slow

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Using AVAST Internet Security 2015 on Windows 7 with AMD Phenom II X4 955 (4x 3,2 GHz).
When the Avastsvc.exe is scanning in Background, the task is using one Core complete (25% CPU Usage) and the mousepointer gets jumpy. Also when listening to Webradio the sound gets stuttering due the high load.

Can someone plese tell me how to limit the scanning process to not use so much cpu?

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I think I have a similar issue. Over the last few months, if I leave Firefox running, but have left my computer for a few hours, when I return I cannot open a new website. Looking at Taskmanager, I find each time that AvastSvc.exe has taken over 50% of my CPU - and has 100% usage of one of my 2 cores. I cannot end the process (not allowed even tho account has admin privs). The computer becomes unusable and I cannot shutdown properly - if I leave it running the system hangs for hours - finally I have to use the power switch to force a hard shutdown. Lately this has been happening most days. I have used Avast Free AntiVirus very happily on this old XP machine until this behavior started recently.

Info:
Windows XP Pro SP3
Firefox 40.0.3 and earlier
Avast Free AntiVirus 10.4.2233 151010-1
Intel Core 2 CPU 1.86 GHz

EDIT:
Restore Factory Defaults (under Troubleshooting) did not fix the problem.