right - the wireless problem was fixed by changing the channel on the router - no idea why SP3 made this neccesary or whether this was a fluke. The PC was still running tragically slow, this is apparently due to SP3 clashes with norton.
I removed norton fully using the norton removal tool on the symantec website, uninstalled all of the other spyware etc software and then dowloaded and installed SP3. Result - PC back to normal. Will reinstall Norton ASAP, if it causes the same problem, I'll ditch norton for something less problematic.