I have read that it would be bad to have two antivirus programs on my computer at the same time.
yes, very important that you uninstall it and then use McAfee removal tool to clar all leftover files that may conflict

see here http://www.avast.com/en-eu/faq.php?article=AVKB11#artTitle

as extra scanner i can recomend Malwarebytes www.malwarebytes.com

if you use removable drives, this is recomended MCShield www.mcshield.net this is a install and forget program…