Just upgraded 6.24 to 7.12 - now nothing works and strange error message.

Hi all.
Just done a clean install ready for a new website so no baggage to bring along or worries about saving a site or db etc.
Installed 6.14 from the sip's CGI library - fine.
Updated to 6.24 via ftp - fine.
Upgraded to 7.12 via ftp (following the instructions in upgrade.txt) and now I get the following error message.
Notice: unserialize() [function.unserialize]: Error at offset 2 of 33 bytes in variable_initialize() (line 943 of /mnt/vol4/home/k/e/keysto/public_html/drupal/includes/bootstrap.inc).
Notice: unserialize() [function.unserialize]: Error at offset 2 of 32 bytes in variable_initialize() (line 943 of /mnt/vol4/home/k/e/keysto/public_html/drupal/includes/bootstrap.inc).
and when I click on any of the links I also get ...
Not Found
The requested URL /drupal/admin/config was not found on this server.
Any help much appreciated otherwise back to a stable and familiar 6.24 I go !
Cheers