Well, I don't think sfc /scannow runs in Safe Mode, but I'm not sure.
I know that many things do not run in Safe Mode and your message has been received by others, for various reasons.
As for backing up...you probably should have been doing that long before your system evidenced problems, it's not mandatory, is merely based on the premise that nothing lasts forever, even happy computers
. It's convenient to have system backups/images when things go wrong.
There is ALWAYS some risk when doing anything...on a computer. Life has no guarantees that innocent, good intentions will have fruition.
But I will say that I have never had a repair install go bad on me (but I'm only one person out of ????), so I would have no fear at all of doing one. IMO, it's a last-gasp measure to avoid doing a clean install of XP and I'm willing to take my chances with it.
For that matter, I don't mind doing a clean install, if I am convinced that is what is called for.
I do whatever it takes to return my systems to operational status whereby I have confidence in their ability to enable me to do what I want to do. And there's enough guidance available that I don't have to worry about not knowing how to do this or that.
MS, XP Repair Install Procedures - http://support.microsoft.com/kb/315341/EN-US/