a few geometry questions...

[TJ Frame]

Hi everyone,
 1) If I have the points A and B, how would I determine point C that lies 
along the slope but is X units in length from the origin (which will always 
be point A)
 I can find the total distance between A and B or the midpoint using the 
distance and midpoint formulas, but I'm not sure how to plug in a specific 
distance value.
 2) I also need to be able to find out where a circle of a given radius 
whose orgin is at A intersects that imaginary line. Given that A will always 
be the origin of the circle and I only want the single intersection heading 
towards point B I wouldn't need to check for all possible solutions such as 
non-intersection etc.
 Any help would be greatly appreciated.. I'm an artist by trade and haven't 
messed with geometry and algebra since high school!
 - TJ