Some advice re detection Trojans

Hi there guys,

 

I was just seeking some advice re my situation here.

 

I use my computer primarily for games and word processing/excel use.  Occasionally I will use it to do financial transactions.

 

I used to have McAfee antivirus (it came with my Dell) on my computer for the last year.  I remember a long time ago it detected a trojan in a jar_cache.tmp file (long time ago) that it reported it had dealt with and deleted.  The computer had since been rescanned multiple times (>40) (I think McAfee has full scans scheduled at least weekly) and nothing popped up.

 

Recently my McAfee subscription expired so I went and bought Norton Internet Security 2012 just yesterday.  I installed, and ran full scan, and voila, it found 9 new trojans, each from jar_cache tmp files in the user/appdata/local/temp folder, which apparently had a date of creation of July 2010.

 

So apparently I've had them for over a year? without Mcafee detecting them?  I have changed my pw's to stuff prior to even knowing about these trojans, and have changed them again.

 

I was contemplating whether to reformat my computer, but I've now realized this will be difficult as I can't seem to find where my product key for Office Professional 2010 went (although it is currently installed on my comp).  Will my computer be secure enough to continue what I have been doing without a reformat?  Or should I adopt the potentially costly reformat?

 

Your advice/input would be great,

 

Thanks