Now this is a curly one mate I can usually use the mouse on that red square to shut down and stuff etc So I am just wondering why you have to use the S key?
Is this machine very old? and has it got a lot of data on it? If not then you should be able to - or at least I would try - tapping F8 on power up when you get the DOS screen it may be worth a try at the repair option.
If that doesn't work and you are not worried about losing data on the machine you can do a factory default - that is you can take it back to an out of the box / shop state by pressing an F key while powering up - different machines have different F keys to do this. Most factory made machines to the best of my knowledge come with a safe recovery partition.
My Google's come up with F3 just tap it rhythmically on power up it should come up with the option to default. It should then go to a reinstall process and all the updates thereafter of course.
Let us know how it goes.
One last thing I didn't ask was do you have nay rescue disk or CD's at all with the machine If not and you manage to reinstall - make one straight away as soon as you get it set up - will save you all this, and when completely set up mate do an image to an external.