"I am not sure why people always think a slow computer means you have a virus"
because they dont really have much knowledge about computers and have fallen for all the "hype" about viruses.
there are others but download CCleaner, a free program easy to use that will amongst other things let you see whats starting up at boot time and then disable those you dont want, or make a list of the programs that are starting and where possible go the preferences for that program and tun off via the program things that are on auto, most common is program updates.
This next one is more hazardous if you dont know what you are doing but theres plent of online advice.
turn off or set to manual, Windows Services that really arent needed on startup and running all the time.
From experience i can tell you Nokia PC suite is a resources hog.
when you do install new software always read exactly what its installing, and make choices , dont just let it install everything automatically, thats how a lot of extra crud gets on you PC.
rather than checking for viruses, you would be better off doing a sweep for spyware and the like using something free like spybot or ad aware, if you do download these utilities dont have them on all the time, just do a periodic sweep or when things seem to be going slow.
no point in defraging your H/D unless it needs it, run the Windows Defrag utility first and then Defrag if needed.