Windows xp wont start properly - Please help!

Hello,
My computer has many important files and pictures that i really do not want to lose on it, but it wont start properly in any mode
When i turn it on, it shows the normal screen which it has always shown, but normally it then went onto the black screen showing 'windows XP' and a moving bar underneath that
now, it shows the same screen but then gives me the option to -
Start windows in safe mode
- safe mode with networking
- safe mode with command prompt
- Last known good configuration
- start windows normally

none of these work!
I have read in several places that you can use the XP disk to start the computer somehow... but could someone tell me if i would lose all of the things on my computer doing this and exactly how to do it?
oh and is there a chance that a virus could have caused this?

Thanks