Google Apps has some of the best spam & virus blocking technology around integrated into Google Apps.
Messages Google believes may be spam are delivered to a folder (well, a label) called "Spam". Messages remaining in this folder for more than 30 days are automatically deleted.
Google will learn what you consider spam. Use the "Report spam" and "Not spam" buttons to train Google spam filters.
Google Apps virus scanning takes action on any messages before allowing a user to download an attachment. Most computer viruses are contained in executable files, so standard virus detectors scan messages for executable files that appear to be viruses. Google Apps helps block viruses in the most direct possible way: by not allowing users to receive executable files (such as files ending in .exe) that could contain damaging executable code -- even if they are sent in a compressed (.zip, .tar, .tgz, .taz, .z, .gz) format.