How often do u need to defrag ur comp, and if u dont what will happen?

Actually, modern computers don't need to be defragmented. That is, unless you are using almost all of your computers hard drive. All defragmenting does is rearrange files to make your computer run unnoticeably faster, so nothing will really happen if you don't. Vista has an analysis feature within defragmenter that can tell you if you should defragment.  more