Can you expand your question somewhat? You need a technique for maintaining performance of a PC, with or without internet access……….by protecting against viruses. Are you asking for a recommendation for a good anti-virus, or a maintenance plan which includes the use of a good anti-virus? Are you looking for strictly anti-virus ( e.g. AVG) or additional protection such as Spybot, Ad-Aware, etc – for periodic maintenance ‘sweeps’? What operating system is this for? The more information you can supply, the better your chances of getting the most appropriate assistance.
To detect the presence of a virus program, you don’t actually need to give yourself a hard time using the command prompt. The best thing you can do is to install a full or trial version of the antivirus software. Make sure to perform full system scan at least once a week depending on your level of use.