MBAM Virus detection. False/Positives?

Hi Geeks.
I'm wondering if you could help with some information. I ran a FULL computer scan (inc Ext HD's) with my MBAM (free version) when I went to bed last night and woke up this morning to find that it had detected 2 Trojan.Zbot viruses in the main C: drive:

Files Detected: 2
C:\Windows\System32\InstallShield\_isdel.exe (Trojan.Zbot) -> Quarantined and deleted successfully.
C:\Windows\winsxs\wow64_microsoft-windows-i..llshield-wow64-main_31bf3856ad364e35_6.1.7600.16385_none_ca61f601a4548b8e\_isdel.exe (Trojan.Zbot) -> Quarantined and deleted successfully.
(end).

I was a little alarmed as I only ran it last week and it seemed clean and did not pick up these two detections.
It is a shared computer yet the other persons sharing the PC don't use it too often.
I was wondering if you could shed any information on whether I need to completely remove these (delete) from Quarantine or whether they are false/positives and I ought to ignore them and restore them from quarantine.

Possibly (un)related and worth mentioning - lately I have noticed that my mouse seems to be quite slow when scrolling through webpages etc and wondered if this could be due to anything specifically virus related..
I use Ghostery when browsing and thought that maybe it was due to this. It seems to slow my browsing down considerably yet when I ask other Ghostery users about this, they say that it shouldn't do.
Any help on the above topics would be greatly appreciated.
Many thanks in advance:
Skinno