Windows XP and later versions give you the ability to create multiple profiles. However, it’s not wise to waste my computer resource to create to users on windows level because I need two profiles running at the same time in Firefox only!!.
This guy explained how to make two profiles running in the same windows profile, everyone has his/her own configuration , plugins, layout, opened pages.. etc.
http://blog.codefront.net/2007/03/01/running-two-firefox-profiles-simultaneously/