You should scan for spywares about once a week..also use the immunity function in spybot search & destroy, its useful.

Pop-ups can be caused by spyware, but more often then not it is just the websites you are surfing offering them.

You can use opera and ditch the 3rd party software to disable nagging pop-ups, you have an option to only open requested pop-ups.

Spyware normally consists of dialers, or just trackers that track your internet usage, and then offer your advertisements on their page tailored by checking your usage statistics.

I doubt you'll need a resident scanner, spyware doesn't pop-up that often now does it? and spyware blaster doesn't and can't be resident because it merely makes your pc "immune" to certain types of spyware.

Remember to always update the software definitions though...