IE7 "Run as administrator" now IE8 flash open and close ERROR

[clumsy_phil]

Running VISTA SP1 desktop machine (Dell Inspiron 530, Q6600, 2GB RAM)

For a while now IE7 would only run when I selected "Run as administrator".
Didn't bother me as I mainly use Firefox, but now I want to convert back to
IE if possible.

When opening IE7 without "Run as administrator", nothing happens, save a
process opening (and seemingly hanging, static on roughly 1MB RAM usage) - ieuser.exe. I'm able to close the process in task manager.

I ran AV checks (Norton Internet Security 2009), Ad-Aware, Malwarebytes. I'm fully up to date with Microsoft updates. I've tried the "Reset" option. I've disabled all add-ons. I've tried the "no add-ons" option. HiJackThis presents no obvious anomalies. I've disabled all but the standard Windows processes at startup in MSconfig. IE7 works in safe mode (which, I believe, is akin to "Administrator" so this is no surprise). SFC reports no problems.

So I tried installing IE8 to see whether that alleviated the problem. Installs fine, no problems encountered.

Slightly different problem with IE8 - when I try to open, a browser window
flashes open and closes immediately. The same happens when I try "Run as
administrator". There is no way I am able to make IE8 work (have not tried
Safe Mode). Again, all add-ons disabled, IE reset in Internet Options etc.

I've seen plenty of forums where people have either one of these problems,
often solved by a Malware scan or System Restore. Since the problem has been
present for a long time I am unable to restore, and the malware scan picks
nothing up. Other people have rogue add-ons - surely the reset will have
disabled all of these?

Other programs I consider it worth mentioning I run on the machine:
Firefox
Norton IS 2009
BlueSoleil Bluetooth software
Java (latest... - have tried uninstalling)
Office 2007
Flash (latest... - have tried uninstalling)
Adobe Acrobat Reader 9
BBC iPlayer / 4oD Kontiki service

Any help is greatly appreciated. All I want is to be able to try out IE8!