If it came with Office 2007 it's probably a Vista machine.
Either way... this should work for Vista and XP. click on the Start menu, then click Control Panel. A new window will open. Find System in that window (it looks like a computer monitor with a check mark) and double click it. You should see all of the system specs on this page--including memory (RAM).
Vista will show a single page with a large Windows logo on it; XP opens a smaller window with a bunch of tabs on it. It should default to the General tab which lists memory towards the bottom.
Hope that helps.