AV = anti virus program There are decent free ones around. Remoc will suggest a good one.

There are organisations who have been sponsored by companies such as the RIAA (Recording Industry of America) who spam the network with rubbish to stop people sharing files. You need to learn to recognise these spam files. Check their file sizes. After a while you will get to recognise them off by heart. Initially it may seem a little daunting to remember them all. But basically ignore any file less than 2,000 KB or 2 MB. There are also a few larger ones to beware of too which is why sample images have been included in the link Remoc gave you.

To get rid of porn spam you can filter it out of your search results automatically using a keyword filter. You initially need to add each word one by one to the filter. Instructions in the link Remoc gave you (both links within that page).