I am trying to generate a report of which version of windows have which service packs etc. I am wondering which file in the windows or system directory has the version information.

Right click on "My Computer" (95-xp) or "Computer" (Vista and up) and then click on properties.

Next to the operating system name you will find which service pack is installed.

You can also go to add remove programs and check "Show Updates" and it will show what windows updates are installed, including service packs. I know the second option works in XP, but I've never tried it in others.

Use Belarc Advisor. It's a free download that does a comprehensive analysis of each PC.