Andrew, Here is what to do. Either you can go ahead and try and install it in safe-mode, which may or may not work because safemode loads in VGA and directx might need to play with your video adapter to install.
If that doesn't work, hit ctrl,alt delete->taskmanager, processes tab. start right clicking on all the non-system process (ie the ones that load under your name) and select delete process tree. Then look at the system process and kill any process that look as if they are being loaded by 3rd party programs.
On the flip side of that you can goto start>run and type in MSCONFIG and ok and then goto startup and just disable everything, reboot.. then try and install. After that re-enable everything.
Lastly and your final fix, would be use a registry cleanup tool. Such as, RegCleaner, which I believe is freeware now, or Norton Utilities WinDoctor.
I'm putting my money on the fact that you have another program running in memory that is causing some sort of conflict. Or a bad combo of programs that are causing a conflict.
If that doesn't work, let me know and I'll come up with a better solution