I still use Kazaa, and have had no problems so far with corrupt files, viruses, etc. As long as you run an updated spyware/adware program and good antivirus (both of which your should anyway IMO ) you should be fine. I've used Napster and it's pretty good IMO, but I'm too cheap to pay $.99 for something I can get for free.
i use napster
plus it downloads a lot faster (about 300kb/sec) and the sound quality is a lot better
it works for me cause ill download like 5-10 songs a month
so its only 5-10 bucksa month which isnt too bad