I've always managed to get round problems like this by re-installing Windows. I'm not referring to a 'full' install (which involves formatting the hard drive and losing all data) but an 'on top' installation which should preserve the majority of data. (The only thing I've ever needed to do, after reinstalling Windows in this way, is to reinstall my anti-virus software. All other data has been preserved).
However, I've only done this using pre-XP versions of Windows. I've never tried it with XP although, theoretically, it's still possible. Instructions are here:
http://support.microsoft.com/kb/315341
Once you've got back to a stable PC, ditch Norton. It's a absolute disaster area. Even if it doesn't crash your PC, it's likely to slow it up. Switch to the free programs which are far better than either Norton or McAfee. The most widely recommended free firewall is Zone Alarm (but Comodo is also popular). For free anti-virus software, use either AVG or Avast. Support those with ant-spyware programs such as Ad-Aware and/or SpyBot.
Chris