excel macro: permission denied (error 70). Strange behaviour

Hi,
I have a problem with Excel macros. I have several files containing macro (saved as macro-enabled files), they run correctly on some windows vista pcs and windows seven pcs.
Recently I formatted two pcs with Windows 7 Pro, installed Office 2007 and performed updates. When I run macro on these pcs, i get "permission denied, runtime error 70", I activated debug mode and I noticed that the error comes at the first line that tries to read a cell; in fact, if I run a macro with only a messagebox popup, it runs correctly, so it seems really some kind of missing permission on the sheet. I checked in excel trusted settings and everything is unlocked and permitted. I tried to disable UAC with no luck. I tried to open excel with administrator account and with normal user account, the behaviour is the same.
So I'm trying to understand what pcs running and pc not running macro may have in common, but they are very different (different op sys, 32 and 64bit, updates level) so I really don't know what to do. The only thing I can notice is that macros don't run in pcs formatted recently. These windows installation has been performed normally, without particular customization or peculiarity.
Pcs are under domain, but there have never been particular policies about excel or other policies that could hurt this subject. Can someone give me an idea?