Any songs you buy have a limit of being on 5 computers. Since you keep reinstalling every time you put itunes back on it will think it is a new computer. It is a bit late now as you will not be able to transfer songs purchased from itunes again. Imported mp3s from CD's etc should be fine. for future reference to avoid using all yoursong activation, go into itunes, and on the store menu click deauthorize computer before doing a reinstall in future.
Only thing I can suggest is Burning your songs to a CD then reimporting them. A waste of CD's yes but when it reimports them they are free of protection (I have to do this to be able to use songs I buy on my phone). To change how files are imported open itunes and go to edit>preferences and click on the advanced tab. Within this click on the importing tab and choose the format you want to import in using the import using: drop down box. You'll have to delete duplicates manually but once its done you'll have files that you can move as many times as you want (and a pile of wasted CD's :o) )