Perhaps you have some other clipboard assistant installed that is making problems ?
It's been a long time since I had this kind of issues and it's all gone since I am not using MS Office extended clipboard anymore.
Perhaps you could try to modify registry, like I mentioned in one of my previous posts.http://support.microsoft.com/kb/207438/EN-US/
I know it is supposed to be for Office 2000, but I read here
that it works on 2003 also.
Perhaps it works also with MS Office 2007, you only need to modify the right key:
For Office 2003:
For Office 2007:
I guess you need to restart the machine after modification for the change to take effect.
One other thing that got my attention while browsing the web is running Excel in Safe mode and thus avoiding some problems with clipboard. See here
on how to do it.