Q: I have a macbook pro 17'. When i switch it gets as far as the start up sound, and then just re-boots over and over, with no screen lighting up.Could you please assist

Could you please tell us what MacBook Pro you're using and which version of OSX.  Answers depend upon what you have.

This used to be an issue in OSX 10.5.

http://support.apple.com/kb/TS1541

I still have no clue what you have.  What version of OSX are you using?  Go to the Apple Menu, open About this Mac, and it should tell you. 

I I still don't know what MacBook Pro you have, but I'll try to figure it out.

It appears that you have a Late 2007 MacBook Pro, making it nearly 5 years old.  If you haven't updated your OSX, then you're using OSX 10.4.  This is old stuff.

Again, look at the link I posted above.  Follow the directions.