this all depends on whether your computer is 64bit or 32bit if it was running vista 64bit befor you downgraded and you installed a 32bit version of xp pro this is why you will have lost your memory as 32bit op systems made by microsoft I.E windows will only see upto 4GB of RAM if you have more than that in your ststem then this is why you have noticed the drop. to gain it back install a 64bit windows.
see this article that explanes all about memory limits in 32 and 64 bit op systems
http://msdn.microsoft.com/en-gb/library/aa366778.aspx
nathan lister
June 2010