You can try to re-install the service pack. That will replace some files, but not necessarily all of them. And if you are missing enough files that SQL Server will not start up in the first place, you may end up having to just uninstall and re-install SQL Server, altogether.
And, of course, you may want to look into restricting the permissions so folks can not accidentally delete these files.
I have not uninstalled SQL Server many times, so I do not know if the data files are left on the disk. It is best to save off backups of all of your databases (master and msdb included), before you start this sort of thing. Once you are certain you can get all of your data back from any mishap, then you can start trying to get SQL Server off the box.
True, its always better to keep the upto date backup of the databases and store them in other location until the resinstallation process is completed.
There aren't any defined steps to reinstall just install the CD and continue as specified in the screen. Its better to do it from scracth by uninstalling current version , remot registery referenced keys, reboot the box and then reinstall SQL.