Where is the Sims save file located?
 

My daughter wants to keep here saved games from the Sims and I'm wondering what File I need to backup when I reformat her computer.

One more thing, I have all expansions installed also but that probably doesn't matter.

Probably in "C:\Program Files\Maxis\The Sims\UserData". There's also a few other UserData directories, like UserData2, UserData3... UserData8. I don't know if these are used but it's best to backup these too to be sure. My best guess is that each UserData directory stands for one neighborhood, with UserData being the first one.