Active spyware protection: which features are worth the resource cost?

[besafe]

It seems like many of the programs that have the most active protections (Spysweeper, Spyware Doctor, Ewido, various HIPS programs), also seem to eat a ton of resources.

So not naming specific programs, but more specific features. Which feautres do you feel are worth there weight in gold?

I like the bootscan, preventing malware from reinstalling after removal. What else?