theirs two ways, easyest way beeing to just put it in quotations..
@echo off
copy "c:\documents and settings\%username%\desktop\file.dll"
the next is ~1 , for example..
@echo off
copy c:\docume~1\%username%\desktop\file.dll ,
most files and folders have pre set ~1 with them, its usualy the first word minus the last letter of the first word plus ~1 , sometimes its the first word combined with a bit of the second and ~1 , cd userse~1
but it could be anything.