As in the link described above, there was this issue in the past where Symantec's Norton reported a false positive about SaferNetworking's Spybot-SD. In conclusion, Norton posted an update which resolved the conflict .
Firefox should be a jiffy. It should start up much faster than IE (in my perspective). Unless you have some dozen themes and add-ons bogging down Firefox. The latest version should do.
The recommended amount of RAM for Windows XP is 512 MB. Say like you have 256MB, that might take some programs longer to open up (maybe by some seconds :P).
Have you cleaned the cache, temp. files, and cookies in your browser?
Also, have you defragmented your drive lately? It should speed things up.