It all depends on the quality of both the scanner and the printer and also what paper you are printing onto. Most modern printer/scanners give good quality results if you use glossy photographic paper.
No your pictures won't be altered by scanning them. The results depend on the quality of the original and the scan resoultion. 200dpi should give you pretty good quality images but you can always up this a bit if you need the scans to be bigger for example if you intend to enlarge the image.
I have just done 7 old photographs.I used Picasa to do the touching up and they are now at least 50% better. I would also suggest that you could use Asda, their scanner and photo process can also lighten and crop the picturesand print out only 6x4. Asda are not expensive!
a better option could be to write your pictures to a disc and get them printed by a professional.
if you can write them to a memory card, you could get them printed off at a local Asda !!
about 25p each i think.
:-)