I wouldn't scan the the tax files with the entire system. Remember that these files are encrypted with a crazy CCH algorithm. This will cause your anti-virus application to try to go through the entire file to see if it finds anything suspicious. That will definitely make your computer lag.
Why don't you transfer all your files over to the computer in the basement and then run the anti-virus on the tax files there. Since you are rarely using the computer in the basement their shouldn't be a problem. I have never had a virus hide in tax files. Since viruses are made for the masses, they usually leach on to common applications such as internet explorer, msn messenger, microsoft word, etc. Not ATX.
However scan them for your safety,
Regards,
Roger