Phoon, actually unistalling the Service Pack and re-installing I think is your best bet. The reason for this is that it will unninstall a lot of stuff (and brake a lot of stuff as well), so then you will get to install updates in order again. You will have to install SP4, and then a lot of updates (I think they are 56, plus the .Net framework), and then DirectX again.
It should take about a couple of days if you can do with the downtime of that PC, but there is a high change that will fix it.