Trojan.Zbot

Every night I like to do scans with Norton and MalwareBytes to make sure my computer isn't infected. Well tonight I ran a Norton scan everything was fine but when I ran Malwarebytes 2 files were shown to be infected with Trojan.Zbot.

 

C:\Windows\System32\InstallShield\_isdel.exe

C:\Windows\winsxs\wow64_microsoft_windows-i...llshield-wow64-main_31bf3856ad364e35_6.1.7600.16385_none_ca61f601a4548b8el_isdel.exe

 

Both were deleted and I'm going to run scans in safe mode then once again in normal mode. And if everything is okay I will change my passwords. What I don't understand is how I couldv'e been infected. I only go on maybe four sites using that computer, my email. FB, a video game site, and amazon. And I rarely open any emails and none if I don't know who they are from.

 

And other suggestions on what I should do? Thank you for your help.