My computer has started running very slowly and internet pages are also often failing to open. My Norton subscription recently expired and i didn't bother renewing it as it was rather expensive. I think I have picked up some virus or spyware. What are the best things (preferably free) for detecting and removing this and keeping it away? Thanks

Go here http://www.ewido.net/en/onlinescan/run/ and run the free scan. I would also suggest downloading Adaware and Spybot from the links in my signature, install, update and run.
Download and install AVG Free from here http://free.grisoft.com/doc/1 and let it run.
Once those scans are done download Hijack this from my sig, run it and post the log in our Syware section for checking. http://www.d-a-l.com/help/forumdisplay.php?f=8

check files that u dont need and delete temperary files