ipod/itunes/windows.need help

i wasnt exactly clear on which section this belonged in...here goes

after my younger (o.d.d.,a.d.d. plauged) brother came to town i let him use my computer freely....whenever he wanted to do i let him...unfortunatly he got a little crazy and long story short...he put a deathly virus on my computer.
i rebooted with windows xp but alll my songs were gone.
so i looked up a tutorial on how to get it back on my comp from my ipod....i got it back but my problem now is that itunes will one day see the files...and the next it wont be there....now theres just an exclamation point to the right of all my songs...meaning they cant find/play music nemore. i still have the files on my computer somewhere...but what can i do to fix this problem completley.
also any free itunes alternative recommendations are welcome

remove all songs from the list(not the computer) then add them all again

winamp with the "ml_pod" plugin works like itunes in that it allows ipod syncing. otherwise, its completely different and its built in music store is crap