There are many potential causes for these sorts of performance problems, so it's difficult to give you specific advice. Any advice we give, without more information, will be fairly hit-or-miss. Try some of the suggestions in the Mac OS X Speed FAQ.
Regarding the whole virus issue, make sure you have installed all the latest updates available in Software Update, and read my Mac Malware Guide. (Note that my pages contain links to other pages that promote my services, and this should not be taken as an endorsement of my services by Apple.)