It's a machine so I have a maintenence routine. I do check the registry from time to time for unusual keys and Task Manager to make sure no tasks are running that I didn't install.
Above all, I don't just click on any old thing.
It has been my belief for a long time that some of the companies who make spyware tools are the #1 purveyors of spyware, malware and adware.
As to the thread topic, I would definitely wait awhile before installing Vista, unless you partion off your HD or have an extra computer to use. Microsoft has also extended support for Windows XP Home edition, so there is literally no rush here.