Whenever I start a scan the mouse freezes.
Only way out is a reboot, ideas?
Is the freeze on both rigs?
Just the desktop. avast had been running fine until recently.
When I remove avast, everything is fine.
Have you tried a different mouse (eg old fashoned PS2 rather than usb connection), or direct rather than wireless? I’m just guessing that the pointer driver may be involved, so something different may be worth a shot to help the investigation along.
Mag beat me to it 
I had a lot of trouble with logitech drivers, mouse using Microsoft drivers was fine
Same sort of problem with KB
I have a wired mouse and reinstalled the mouse software with no avail.
In task manager, it seems to me that avastsvc.exe is bouncing around and the cpu=95-100%. I’m no expert, but it seems like a memory leak.
Have you tried using the mouse without installing its software?
You might want to try and repair Avast and make sure that there are no other AV programs or remnants of them on your HDD.
Are also you running any realtime malware programs?
Nope.
Do you have your Browser or any other programs like OO open when you scan?
If you do close them and try the scan
You may also want to Google “mouse freezes”
Depending on your OS; i.e. XP or W7 there are ways that you can have Windows check for problems and fix them.
Does the mouse freeze on only certain scans like Complete System Scan or on all scans?
All scans.