The free version of Avast! is good but some antiviruses aren't meant for some systems = problems. If and when that user installs it and it crashes again, MSE should be a permanent switch.
BTW yes, multiple antiviruses do create conflicts. Is it bad to run multiple antivirus programs?
Afraid of virus? Check Good and Free system security combination.
A few good & small scanners
TDSSKiller: Anti-rootkit utility TDSSKiller
, ESET online scanner: Free Online Virus Scanner | ESET
Windows Defender Offline
and they're free