Hi
I have spent all day on this one tried all kinds of advice (eg deleting iconcache.db, changing from 32 to 16 bit display and back etc....). Extremely frustrating! but I finally cracked it!!
Here was the problem: I think some over-zealous cleaner tool (TuneUpPortable

) trashed my Installer directory:
I had started to look at my file associations in the registry.
Options - File Associations was very helpful here with info about HKEY_CLASSES_ROOT and how the defaulticon is set for each type of file.
Well I found out that the microsoft word icons are not obtained from the actual WINWORD program but from this folder here:
HKEY_CLASSES_ROOT\Word.Document.12\DefaultIcon
C:\Windows\Installer\{90120000-0030-0000-0000-0000000FF1CE}\wordicon.exe,13
So I had a look, and not only was the file wordicon.exe missing, the whole folder was gone! In fact my whole installer folder was looking pretty desolate
Luckily I have another computer with similar setup so I was able to copy across a whole bunch of folders that were missing. It is possible to guess what program uses which folder (eg I could see an icon used by excel or some files called pdfxxxx etc...) and if I had this same progam installed on my "broken" computer and the folder was missing, I just copied it across. At worst, you can check the registry like I did above for any that you dont find...
It Worked!! Heres the Office 2007 folders that should be in the is install folder (at least for me - the number may be some serial code or something) - seems like they all end in "OFFICE":
{90120000-00D0-0409-0000-0000000FF1CE}
{90120000-001C-0409-0000-0000000FF1CE}
{90120000-006E-0409-0000-0000000FF1CE}
{90120000-0020-0409-0000-0000000FF1CE}
{90120000-0030-0000-0000-0000000FF1CE}
And this was my Adobe Acrobat 9 program:
{AC76BA86-1033-F400-7761-000000000004}
Hope this helps someone
BJ