1-Download imageJ from http://rsbweb.nih.gov/ij/. There are MAC, Linux, and Windows 32 and 64 bit versions.
2-Install it. With Vista install it in a directory like your documents or downloads. Then it will have full write permissions. It's a Java applet and works a little different that most windows programs.
3-Launch it. You will see the small Image J box. (For don't-want-rewrite-the-software reasons you can't resize it. On an old VGA screen it looked big.)
4-Click on edit-options-plot profile options. Out of the box, ImageJ autoscales. Since there are 256 gray levels (0 black to 255 white) set the minimum Y to 0 and the maximum Y to at least 255. I use a fixed scale if I'm making comparisons since it makes the differences more obvious.
If you are doing something else, set it any way you want. This box only effects how the plot will look. Click OK to set the scaling
5-Click on file and open an image.
6-Right Click on the line icon box (the fifth over). With version 1.42 you can now draw straight, freehand and segmented lines. Pick a type and drag a line in the image you opened.
7-Hit control+K or the MAC equivalent. Or go to analyse-plot profile in the menu bar. ImageJ will calculate and plot out the grey scale values under the line. If you drew it in an area where there is little detail except for noise you will have a noise profile.