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And did they, or you, test with that fresh installation, without restoring anything from a backup?

Third-party system modifications are a common cause of usability problems. By a “system modification,” I mean software that affects the operation of other software — potentially for the worse. The procedure will help to identify which such modifications you've installed, as well as some other aspects of the state of the system that may be pertinent.

Here's a brief summary: Copy a line of text from this web page into a window in another application. Wait about a minute. Then paste some other text, which will have been copied automatically, back into a reply on this page. The sequence is: copy, paste, paste again. Details follow.

You may have started the computer in "safe" mode. Preferably, these steps should be taken while booted in “normal” mode. If the system is now running in safe mode and is bootable in normal mode, reboot as usual. If it only boots in safe mode, proceed anyway.

Below are instructions to run a UNIX shell script. It does nothing but produce human-readable output. However, you need to be cautious about running any program at the behest of a stranger on a public message board. If you question the safety of the procedure suggested here — which you should — search this site for other discussions in which it’s been followed without any report of damage. If you can't satisfy yourself that these instructions are safe, don't follow them.

Triple-click anywhere in the line of text below on this page to select it:

clear; PB="/usr/libexec/PlistBuddy -c Print"; PR () { [[ "$o" ]] && o=$(sed 's/^/ /' <<< "$o") && printf '\n%s:\n\n%s\n' "$1" "$o"; }; PC () { o=$(egrep -v '^[[:blank:]]*($|#)' "$2"); PR "$1"; }; PF () { o=$($PB "$2" | awk -F'= ' \/$3'/{print $2}'); PR "$1"; }; PN () { [[ $o -eq 0 ]] || printf "\n%s: %s\n" "$1" $o; }; a=$(id | grep -w '80(admin)'); [[ "$a" ]] && sudo true && r=1 || r=; { [[ "$a" ]] || echo $'No admin access\n'; [[ "$a" && ! "$r" ]] && echo $'No root access\n'; system_profiler SPSoftwareDataType | sed '8!d;s/^ *//'; o=$(system_profiler SPDiagnosticsDataType | sed '5,6!d'); fgrep -q P <<< "$o" && o=; PR "POST"; o=$(($(vm_stat | awk '/Pageo/{sub("\\.",""); print $2}')/256)); o=$((o>=1024?o:0)); PN "Pageouts (MiB)"; s=( $(sar -u 1 10 | sed '$!d') ); [[ ${s[4]} -lt 90 ]] && o=$( printf 'User %s%%\t\tSystem %s%%' ${s[1]} ${s[3]} ) || o=; PR "Total CPU usage" && o=$(ps acrx -o comm,ruid,%cpu | sed '2!d'); PR "Max %CPU by process (name, UID, %)"; o=$(kextstat -kl | grep -v com\\.apple | cut -c53- | cut -d\< -f1); PR "Loaded extrinsic kernel extensions"; o=$(launchctl list | sed 1d | awk '!/0x|com\.apple|org\.(x|openbsd)|\.[0-9]+$/{print $3}'); PR "Loaded extrinsic user agents"; o=$(launchctl getenv DYLD_INSERT_LIBRARIES); PR "Inserted libraries"; PC "cron configuration" /e*/cron*; o=$(crontab -l | grep [^[:blank:]]); PR "User cron tasks"; PC "Global launchd configuration" /e*/lau*; PC "Per-user launchd configuration" ~/.lau*; PF "Global login items" /L*/P*/loginw* Path; PF "Per-user login items" L*/P*/*loginit* Name; PF "Safari extensions" L*/Saf*/*/E*.plist Bundle | sed 's/\..*$//;s/-[1-9]$//'; o=$(find ~ $TMPDIR.. \( -flags +sappnd,schg,uappnd,uchg -o ! -user $UID -o ! -perm -600 \) | wc -l); PN "Restricted user files"; cd; o=$(find -L /S*/L*/E* {/,}L*/{A*d,Compon,Ex,In,Keyb,Mail/Bu,P*P,Qu,Scripti,Servi,Spo}* -type d -name Contents -prune | while read d; do ID=$($PB\ :CFBundleIdentifier "$d/Info.plist") || ID=; ID=${ID:-No bundle ID}; egrep -qv "^com\.apple\.[^x]|Accusys|ArcMSR|ATTO|HDPro|HighPoint|driver\.stex|hp-fax|\.hpio|JMicron|microsoft\.MDI|print|SoftRAID" <<< $ID && printf '%s\n\t(%s)\n' "${d%/Contents}" "$ID"; done); PR "Extrinsic loadable bundles"; o=$(find /u*/{,*/}lib -type f -exec sh -c 'file -b "$1" | grep -qw shared && ! codesign -v "$1"' {} {} \; -print); PR "Unsigned shared libraries"; o=$(system_profiler SPFontsDataType | egrep "Valid: N|Duplicate: Y" | wc -l); PN "Font problems"; for d in {/,}L*/{La,Priv,Sta}*; do o=$(ls -A "$d"); PR "$d"; done; [ "$r" ] && { o=$(sudo launchctl list | sed 1d | awk '!/0x|com\.(apple|openssh|vix\.cron)|org\.(amav|apac|calendarse|cups|dove|isc|ntp|post[fg]|x)/{print $3}'); PR "Loaded extrinsic daemons"; o=$(sudo defaults read com.apple.loginwindow LoginHook); PR "Login hook"; o=$(sudo crontab -l | grep [^[:blank:]]); PR "Root cron tasks"; }; o=$(syslog -k Sender kernel -k Message CReq 'GPU |hfs: Ru|I/O e|n Cause: -|NVDA\(|pagin|timed? ?o' | tail -n25 | awk '/:/{$4=""; print}'); PR "Log check"; } 2> /dev/null | pbcopy; exit

"Avast" is perhaps the worst of the whole wretched lot of commercial "security" products for the Mac. It's worse than the imaginary "viruses" you were worried about when you installed it. Not only does it fail to protect you from any real danger, it throws false warnings, destabilizes and slows down your computer, and sometimes or always corrupts the network settings and the permissions of files in your home folder. Removing it may not repair all the damage, and neither will Disk Utility or even reinstalling OS X.

Back up all data, then remove "Avast" according to the developer's instructions. Reboot.

If you tried to remove Avast by dragging an application to the Trash, you'll have to reinstall it and then follow the instructions linked above.

If you still have problems after following those instructions, post again.