urgent… invalid system disk !

hi. i’m having a problem with my laptop. whenever i turn it on it says:

invalid system disk
replace the disk, and then press any key

the laptop has two users and is running on windows xp pro. i really need to solve this… i think the only solution right now is to format… but if you guys have a way to access my files… not only does my computer contain my thesis and projects, it also contains my aunt’s office files and documents.
is there any way that i can still access the files so that i could back them up?

thanks, more power, and keep up the good work. ^_^