<<How can I be sure it's a system file or a program file?>>
Program files will in the C:\Program Files folder, while most system files will be in C:\Windows folder.
The files contained under C:\Documents & Settings are also system files. But the contents of these folders can generally be moved to another drive or disk (e.g., Favorites, email, cookies, contents of My Pictures, My Documents, etc.
Any files under C:\i386 are system files.
Any files under C:\System Volume Information are system files.
Space taken up by Recycle Bin deletions...is not restored until the Recycle Bin is emptied.
C:\Pagefile.sys is a system file whose size can be adjusted.
A more detailed list: http://technet.microsoft.com/en-us/library...57124.aspx#EIAA
A tool that you can use to view the files/folders which are largest on your system: TreeSize Pro Trial - http://www.jam-software.com/treesize/