A desktop top computer - you mean a really good PC? or just a normal one?
What are the current spec's (CPU + RAM, how old etc)
Why are you working in offline mode, and with an antivirus program - have you got problems?
Your best defense against virus's to start with is not to click on anything suspicious, and avoid sites / downloading things that you dont trust.
It depends on how you are judging 'best' in antivirus software, low cost, or are you all about protection? Depending on your current specs, we can maybe determine why the computer is being slowed down and try other solutions to speed / clean it up first.
If you dont post again or answer any questions from above, Mcafree / Norton are ok programs, but again a little performance intensive.
For less drain on your performance maybe AVG / Avast! / Microsoft Essentials could be worth a shout.