New2me HP Notebook hlp w/possible virus trojan

I was given a used HP Notebook and previous user visited a lot of naughty sites. I used sys recovery to reset to factory settings but am afraid there may be a trojan still on the computer. I am a novice so anyone who may help me, please send step by step instructions. Here's the little bit of info that I know to get:

HP 2000 Notebook PC
Windows 7 Home Premium, Service Pack 1
Intel Pentium CPU B970 @ 2.30 GHz 2.30 GHz
RAM 4.00 GB (3.90 GB usable)
Sys type 64-bit Operating System
No Pen or Touch Input avail for this display

I appreciate any and all help!!!