See "To extend a simple or spanned volume" at
<<ms-its:%windir%\Help\Diskconcepts.chm::/dm_extend_volume.htm>>
a.. You can extend a volume only if it contains no file system or it is
formatted using NTFS. You cannot extend volumes formatted using FAT or
FAT32.
a.. You can extend a simple or extended volume only if the volume was
created as a dynamic volume. You cannot extend a simple or extended volume
that was upgraded from basic to dynamic.
a.. You cannot extend a system volume or boot volume.
There are likely third-party utilities that will do this for you. You also
have the option of moving directory branches off of C. It is possible to
move Documents and Settings; see "Cannot Move or Rename the Documents and
Settings Folder" at
<
http://support.microsoft.com/?id=236621>. It is also possible to move
Program Files, but that's probably not supported, nor easy.
And of course, the normal rules of disk maintenance apply. You can delete
%winder%\temp, %temp%, miscellaneous temporary (dir /ad /s /b temp), and
temporary Internet files. You can evaluate deleting older Service Pack and
patch directories from Windows. You can prune away unneeded profiles. You
can uninstall Windows components that are less important than diskspace,
such as document templates, wall papers, and screen savers, although those
consume little space. You can convert to NTFS if this drive is FAT; that
alone generally gives one some breathing room. If it is NTFS, you can
compress some branches or the whole disk.