I have the same kind of broken installation, and no clue about how to recover.
I was trying to remove an older Cisco VPN client, and (perhaps mistaken) hit
cancel to stop the uninstall to remove it later. Now I can't remove it, and
get the error message you cite. I also cannot "Repair" which gives a similar
error message in a different widget. It really bugs me that there is no way
to explicity resume an installation (which Vista claims needs to be done) and
no way to determine what it is that is being installed. It's a terrible lack
of transparency.

"sean in d.c." wrote:

> I am unable to install or uninstall any programs in Windows Vista. If I
> attempt to do so, I get the following message:
>
> Another program is being installed. Please wait until that installation is
> complete, and then try installing this software again.
>
> I get this error if I try to install a program using a CD, DVD or download,
> and I get this error if I try to uninstall a program from the Control Panel.
>
> I have rebooted several times, yet this problem persists. Any suggestions?