You don't need to pay for a subscription service to any spyware detection tool.
Here are two free products, regularly updated, that are well-regarded within the security community:
Spybot Search & Destroy
Spyware Blaster
AdAware is also good although their updates have been sporadic recently and some people have had problems with the latest version producing false positives.
ETA - Forgot to answer your other question. Spyware does more than simply track your usage. Apart from the privacy concerns, it is usually poorly written and can slow down or crash your system. Worse, much of it is malevolent and provides an open path for trojans and other nasties. So yes, you should rid your computer of it and scan regularly for new infestations.[This message was edited by Jeff Agnew on 30 December 2003 at 04:35 AM.]