i dunno if this has been dealt with b4, but anyways...
3 good anti virus softwares which are downloadable free of charge are
go to the page ^^ and on the right hand side bottom, you'll see a "download" bar.
2) Avast4! Home
just go to the ^^ page and click on the "english" version.
this is ofcourse the most well known one. so just google "avg free download" and then get it.
hope it helped. i got the first 2 names from b.b.c.'s "click on line" and i have been using AVG for about 2 years now with no problems.
if anyone else knows about other good and free "anti virus" packages, then please let us know.