Can't enable "Show as conversations" in Outlook 2010

Outlook Experts: I'm having a strange problem with the RTM version of
Outlook 2010 - I can't enable the "Show as conversations" option from the
View tab of the menu. I can click the checkbox, but whenever I click on
anything (e.g., another mail message), the setting disappears. The
"Conversation Settings" menu is disabled. Some details:

1) The problem appears to be machine-specific. I tried another user
account on the same computer, and had the same issue. I also tried creating
a new mail profile and have the same issue.

2) Using the same PST file on another computer works fine, and three other
machines that are running Outlook 2010 seem to have no problem with this view.

3) I had a post-beta/pre-RTM build of Office 2010 installed on this machine
for test purposes. I did not uninstall it before installing the RTM version
of Office 2010. I understand that this is a big red flag, but I've done the
same on three other machines and don't have any problems at all.

4) I'm running the 32-bit version of MS Office Premium Plus (MSDN) on
Windows 7 Ultimate x64.

So far, I have tried the following:

1) Comparing both the HKCU and HKLM keys for Outlook between this computer
and one on which the conversation view works. I can't see any differences
and can't immediately see any relevant portions of the settings.

2) Started Outlook using numerous startup switches, including /safe,
/nocustomize, and /noextensions and /regserver.

3) Ran ScanPST on the Outlook file (errors found and fixed); I doubt this
is relevant, but I thought I'd mention it.

I haven't yet uninstalled Office 2010, but that's an option. I would
probably delete all relevant Registry keys, but I'm not sure if there are
some that I'm missing.

Does anyone have any idea what I can do to troubleshoot this further? Any
help or suggestions would be greatly appreciated!

- Anil Desai