It's extremely rare for newer software to use less resources
It happens every once in an extremely great while, but it's almost unheard for a new version of anything to use less resource. I'm sure IE8 will use a bit more, but if you don't have it, you're totally out of luck getting into the MS site and getting the rest of what you need. If you're so tight on disk space that you can't afford another .1 GB or so, then you need to get a larger hard drive.
You say you want to change the boot sequence so that it doesn't boot from any disk, but then you wouldn't be able to boot Windows which makes no sense I'm afraid. Changing the boot sequence requires you to go into the BIOS which is why you've been told about that.
Once you install IE8 and get up to date with Windows updates, you should have whatever versions of .NET you need.