NOD32 is very sufficient as a primary defense, and if you are slow already, these add-ons (if realtime scanners are left running) will only make you slower.
I am also inclined to check RAM (and CPU) to be certain that the machine is as you gave it to the shop, again as others have suggested.
Another thought crosses my mind- Were you set up previously to hibernate rather than using a full shutdown/start each time? Most Vista users find the hibernate option to be much more comfortable... Your boot time restoring from hibernation would be mere seconds (maybe 30/45 secs YMMV), and you would only experience a full restart if Windows (or certain types of programs) required it.
I recently had some thing that was hijacking google clicks and displaying male enhancement banner ads on every page. Adware and SuperAntiSpyware were able to point it out, but do nothing about it. McAfee was already disabled by the thing. The only product I found that could remove it was adwarebytes.org.