your best bet is
http://www.ccleaner.com/download
(it IS free - just doesn't look like it)
it will get into all those important little places that are safe to delete
just use the default settings ... if you have lots of sites you log into - untick cookies.
if you don't know where it is - it's best to leave it alone..
xml files are used by your web browser ... so should be safe to delete (if in temp internet folder)
HOWEVER - they are also used by an increasing number of progs as configuration files ... and these most definitely are not safe to delete.
if you must delete files I tend to move them to a zip file
and then after a couple of weeks without problems delete them