if you have already instsalled SQL, you can change the collation using setup. See BOL under 'how to install sql server 2005 from the command prompt' then section 'To rebuild system databases and specify a new system collation'

You will then need to re-apply any upgrades you previously had.

old backups of the system databases will no longer be restorable so script out any logins etc from master.

have everything backed up\scripted out before you start