nailed that one, that's exactly what's happening (interpolation, it's the area that's between the pixels, as pixels are shaped like stop signs, so when you put them together, there's an "empty" spot in between them that the program has to guess what color or information should go there). Photoshop does really good with guessing what's in there, depending on the method that you use to do so. Photoshop can be yours for the low low price of about $600
One thing about your post that I noticed is that you are scanning at 600 dpi....I should think that it's "overscanning" IMHO. One thing to keep in mind when scanning an image that you're not going to print and will only use for viewing on a computer screen is that most monitors only display at 72 dpi. Sooooo...if you're scanning in at 600 dpi, and will only view the image at 72 dpi....big difference! Also, scanning at that high of a dpi will pick up every little thing that's on the image, including dust and hair. You can manually lower the resolution of your scan right from within the scanner itself. Also, if you're enlarging it, you should be able to adjust the output size (enlarge to etc. etc.) from within the scanner settings. This will help you to avoid taking an image that's normally 2 inches wide by 3 inches tall and literally stretching it to be 8 inches wide by 10 inches tall (out of proportion, I know, but it's just an example
), which will stretch the pixels in turn making the entire image look bad.
Now, on the other end of the spectrum, scanning at to low of a setting (low dpi), then when you look at it, it's going to look bad anyways, simply from a lack of quality. I tend to scan at 80 dpi for the web or 200 dpi if I'm going to print.
Best thing to do if you're wanting to scan images to display on your computer is to find them on the internet, as they'll already be designed and ready for "web use", so they're pratically guarenteed to look good! tg1911
has provided links that should be able to help you find what you're looking for!