Short answer: You can't. Microsoft is going to keep pushing SP3 on account of it being chock full of security fix goodness.
If you installed it, and it caused problems, it's most likely due to malware damage. Since you use IE, and IE6 at that, it's practically a given that your system is crawling with malware. That would be probably the single biggest cause of problems with service packs.
How do I remove an automatic update that's been downloaded but not installed, and block it from being downloaded again in the future?
I had installed SP3 for IE6, but it caused problems, interfered with my burners, etc., so I restored to just prior to the instalation.
But now it's been downloaded again, and although I unchecked it under "custom install", I now find it's trying to install it everytime I go to turn off or reboot