you should backup your entire registry before you install IE6.
start>run>regedit>registry>export registry file>export range=all>
then name the file something like "before ie6" (without the quotes) ans save to a directory and to a floppy if it will fit or a CD if you have a burner.
That way if IE6 hoses your instalation and you cannot get it fix correctly, all you would have to do is use internet explorer repair tool to revert back to your previous IE version. That repair alone might fix the problem if you have one, but the repair tool alone doesn't always help. That's where the registry backup comes in.
Since it is too hard to tell every single entry in the registry that IE effects, so you could edit the registry to fix it, all you have to do is merge the good registry file to the existing registry and BAM everything will be fixed.
If you changed a setting after you exported the registry and then merged the exported file into the existing registry, the setting will revert to the one in the exported registry.
I myself have before and after registry exports for every program I install. Plus on the first boot of my Windows install, I export the entire registry and save it as "first boot everything cool"
hope that helps
oh I should mention that editing/merging the registry can be hazordous. So always backup first and you will be fine.