well the dvd+rw might not be surported the older towers are finicky about which drive they wnat to boot from especially older drivers newer drivers are easier to boot from

i bought a lite on dvd writer in 2005 and my G4 tower booted just fine off it

the update might have borked the compatibility between the drives and your mac ( unlikley but entirley possible )

your cd's might be damaged

or your hd is hosed

i think the first two are more likley

oh btw check your ram

a broken ram moduel can play al kind of havoc my quicksilver refused to complete its install because of a broken 128 mb stick