I'm glad to see what Richard checked worked out as it did and the problem he suspected wasn't the case - though I feel certain that something is trapped in Windows Installer and causing this behavior.
So SFC looks fine (which is good because it means most of your system files - those it checks - have no corruption, and bad because it gets us no closer to an answer - but I much prefer the results you got).
How are you doing with installing the newest version of Installer and do you have a genuine Vista Installation Disk or just a Recovery Disk (or how would you re-install the system if that was necessary)? If upgrading the installer doesn't solve the problem (or can't be done because of problems with the installer or other reasons) and if you won't consider the Cleanup Utility (and to be honest, while it can do a great job when it works properly and solve problems like this and has helped many people, it does have some risks and there have been some cases where it has caused more harm - not many, but some), then the most logical next step is a system repair/upgrade but for that we need a genuine Vista Installation Disk which is why I'm asking. If you don't have one, is there anyone you can borrow one from - it just needs to be the same bit-size as they all contain all versions of Vista.
I hope this helps and look forward to your reply so we know where to go. If this doesn't work, then we may need to consider a clean boot after you backup your data. At that point, you'll have nothing to lose by trying the Cleanup Utility because even if it messes things up worse, you'll still be re-installing so it won't make any difference - and it may instead solve the problem. But let's try these other options first and see if we can resolve the problem without needing to go this far.
Good luck!