Allow me if you already knew the following (I have no idea whether you are an experienced user or not):
(1)
it will not un-tar any tar archives created with a newer version of Tar
This indicates that you created the archives on a different machine and transfered them to the Tiger Mac. Are you sure the archives were not broken during the transrer? For example, if you used FTP for transfer, are you sure you used binary transfer mode?
Where did you created the archives? On a Leopard Mac?
(2)
If you are extracting from gziped tar archive (xxxx.tar.gz), then please make sure to add 'z' option when extracting, i.e.,
tar zxvf xxxx.tar.gz
The tar commmand installed in Leopard is based on GNU tar 1.15.1, which is so clever that it can automatically recognize gzipped archive, so
tar xvf xxxx.tar.gz (without 'z')
also works on Leopard, but not on Tiger; you must explicitly add the 'z' option like 'tar zxvf ....' on Tiger.
(3)
If 'tar zxvf' doesn't work, please try unzipping manually, i.e.,
gunzip xxxx.tar.gz
tar xvf xxxx.tar
or
zcat xxxx.tar.gz | tar xvf -
(4)
Please note that the versions of tar command installed in Tiger/Leopard are not identical with those distributed from GNU tar site. Apple's tar is extended in a way that it can handle resource forks and extended attributes. If you want to extract resourece forks/extended attributes then you will need Apple's tar, whose source can be downloaded from
http://www.opensource.apple.com/
but I don't know whether building Leopard-version of tar on a Tiger Mac is easy or not. Or you may try Fink or MacPorts if they have newer tar for Tiger.