Walgreen's Picture Quality Issue

I'm an amateur photographer, I'm only 16 so I don't know too much about the business, and I took some photos of a family and after I edited the pictures (I didn't edit them much, just adjusted the lighting using Picasa 3) I put them all on a disc and gave them to her to do what she wanted with them. Well she said that she went to Walgreen's to print them off, and she said the quality looks horrible on them, except for the wallet sizes. She said that it wouldn't even let her print some sizes off due to quality issues.
I need advice on what I'm supposed to do. I use a Nikon D80, so the pictures are already high quality, and I can't print them off for her myself, but I've never had this happen to me before with any of my pictures, but I always just go to Walmart. Is it just Walgreen's, or my pictures? Help!
Thank you so much!

Added (1). Like I said, I'm only 16, I'm an ameteur, I don't know very much about these things yet. I didn't know if the type of camer affected the size.
I'm using Picasa because I don't have the money to get Photoshop or to buy the Nikon software. I have it downloaded on my computer, and it works great for my level of photography, thank you very much.

And I'm just confused on this whole situation because I have printed pictures off at Walmart (and they were edited way more than these) and all the camera settings were the same, but they weren't bad looking at all. I got them blown up big too, and they looked great.

And the file size is?

One thing to check is the PPI - pixels per inch. Many cameras default at 72 ppi for jpgs which is fine for web viewing. If you print, having a PPI of 300 is good. The smaller the print is, the less noticeable this would be.

It obviously is a size / resolution issue. Is Picasa an online editor or do you have it downloaded on your computer? If online, it may be drastically reducing the size of your photos by default and that in turn may be what you have put on the CD.

Do you not have a copy of the original files you took? Check them over again and see what the size and resolution is on the photos BEFORE editing them in Picasa.

If you took the photos at your camera's largest size / highest quality settings AND you have a copy of the original files, this should be an easy problem to rectify and simply put a copy of correctly sized files on another CD.

Just because you are using a nikon d80 does not mean the pictures are high quality (did you shoot at the highest resolution on your camera, or was it on some sort of "email" mode?).

Also, after you edited them, did you save them using the highest jpeg setting, or the lowest? If you used the lowest, then the quality degrades.