Mark88,
I don't know why you want to enable the event logging for IE8 (you need to do this if you want the events to show up in the event viewer. In any case, whatever reason you have to want to see the events, you need to download and install the Microsoft Application Compatibility Toolkit from this link:
On the last line of the link above there is a link for the Microsoft Application Compatibility Toolkit 5.5
http://www.microsoft.com/downloads/details.aspx?FamilyID=24da89e9-b581-47b0-b45e-492dd6da2971&displaylang=en
After installing it, you should be able to see or create the path in the screenshot below. If you are not comfortable working with the registry, modifying or creating new entries, I would strongly advise you to leave it alone... You can incapacitate your Windows installation completely (and normally... all the events reported in the event viewer regarding IE8 are mostly useless unless you know what you are doing which is why it is not enabled by default in the first place).
My advise is for you to take the normal precautions anyone should take every time you install a new program or when you are going to do changes in the registry: Backup your data and backup your registry.
Bits ...
