[arch-general] Error for extract archive .tar

[dario]

Hi! i have a problem for extract an archive .tar, because when write
"tar -xzvf archive.tar.gz" i get the messenge:
gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now

i have installed tar and gzip, and would install 7zip, but i can't.
What is the problem?
Thank you very much

-- 
Dario

PD: sorry my english, your understanding