ERROR 80 CANNOT BOOT

i was using my Ubuntu 8.04 when after downloading recommended updates from the update manager
the suddenly the computer hanged.
the computer did not respond to mouse movements, keyboard buttons even ctrl+alt+del
i turned off the computer using the power button

when i turned it on again
it said there was a problem with the Graphical interface
then i turned it off again because I wasn't able to use it

the next day, i turned it on again
this time the computer gets stuck for a while after detecting my optical drive
then after it starts to work

it would go to a screen saying insert a proper boot device or press any key to retry

i tried changing my boot priorities in an attempt to boot the computer from my cd.
the try ubuntu without instaling anything works fine

however

after turning it off properly using the shut down button
i tried to turn it on again
i get the same ERROR MESSAGE

i tried to install ubuntu again and when it started it said error about a partition on my hard disk
now Im stuck unable to use both my Linux Ubuntu and Windows XP.

the error 80 appeared when i was trying to reinstall ubuntu

this are my desktop specs

Intel E6550 2.33Ghz
asrock conroe 1333-667 mother board
2x 1gb kingston ddr2 667
seagate 200gb SATA HDD
geforce 7300gt 256 mb 128bit ddr3