Can’t Access Network drive C?

I am working in Windows XP, and others my colleagues use Windows 7 and someone else uses Windows Server 2003 or Windows 7. Sometimes I need to access network drive such as c drive, d drive e drive etc. without manually sharing this drive. Usually I can access all network drives but can’t access this on the computer which is running on Windows 7.
I try to access Windows 7 computers C drive by administrative password, but can’t access. So please help me how to access? My network is OK and no firewall is on that computer.
