Yes it's quite clever!
You've hit upon a syntax for creating an NTFS
alternate data stream
and I think you will have created an alternate stream named :sample.txt which is a complete copy of the original file but hidden from normal view.
A bit of background is available in
http://en.wikipedia.org/wiki/Fork_(file_system)[
^] especially in the links at the end of the article.
As XP doesn't contain anything for viewing or deleting these I recommend you download a tool such as
AlternateStreamView[
^] which will let you see what you've done.
Suprisingly Notepad will let you view a text format
alternate data stream
. Try typing
Notepad :Sample.txt
in a command window to see what happens.
Alan.