You'll get a lot of conflicting advice on this so be prepared to play Solomon and sort through it.
Since I get to be first, I hope I can get you going before it gets ugly.
I think the water got muddied when SP2
or some later HotFix changed the scenery on the SFC applet. That's just a guess though. I remember when it offered a "Browse" window as an alternate selection when it asked for the CD. When that happened, it was only necessary to browse to the I386 folder on the root of the system drive and things went smoothly from there. No further demand for the CD was made.
However, and you knew there would be a however, the last time I ran sfc /scannow, it asked for the CD but there was no alternate window for browsing. If that be what you are faced with, you will have to edit the Registry as I did. There is a key that has recorded the original install path and setup path. If that key has the correct data string, the CD request will defer to that location.
The key is:
By all means, export a copy of it before editing. Post back what the data strings you find for SourcePath
and Installation Sources
I can tell you one other thing, it's easier to borrow an XP install CD than grind through this sometimes. Keep that in mind when you get frustrated.