I have installed Windows 7 professional on machine, When I am trying to Access a windows xp machine it says "Logon failure, unknown username and password bad"

So you have two partitions with each Windows is installed? or is it a virtual machine? "Everything we do is dictated by motive."

In what way are your trying to access the windows XP machine? Through remote desktop? if you are and they arnt part of a domain, you will need to log onto it using a user account on the windows xp machine. also you wont be able to remote desktop onto it if it is windows xp home as it doesnt allow remote desktop connections (windows xp professional does if you enable it). Or accessing a network folder? again if its not part of a domain, you will need to use a username on the windows xp machine. Basically if the windows xp machine isnt part of a domain, you will need to access it using an account that exists on that XP machine, and not the account you use on the windows 7 machine.