That's really strange, the installer should leave files like that alone since they are not part of the install package (msi).  And WebLink should just need read rights to that location, which should be granted by default.  You might want to add the IIS_IUSRS group (unless that's what you mean by IUSR), and if that doesn't work I would use a tool like Process Monitor to see a little more detail about what is failing.