Did you install something like PC Defender or something with some similar name?
PC Defender (recently ported to the Mac as Mac Defender) and it's numerous clones are rogue security that DON'T report the real state of your computer. Instead, they resort to a fear inducing tactic by telling you that you have numerous infections, errors and problems that absolutely need to be taken care of.
Those rogues ared trojans that attempt to infect your computer with some very real virus, and have you pay good mony in addition to that.
First, download a REAL avtivirus like AVG or Avira. Both are available for free and are very good.
Have it perform a full scan as proposed upon successfull instalation.
Next, for the possibility of some genuine errors, I STRONGLY recomend CCleaner from Pyriform. It's a great free application than can safely remove almost all junk and leftover files heating disk space. It can also scan your computer to find several kind of errors and repair them. The repair step offers you the chance to backup your system at it's present state in case that the repair may actualy cause some problem.
Electro, August 2011