My suggestion is to see if you can turn off Norton and go with a freeware anti-virus first (such as AVG or Anti-Vir - both available from download.com), then reboot and see if anything breaks.
Also see if you can find out about the programs / processes that run when the PC starts, and try to disable as many of them as possible.
This way you can isolate and narrow down the source of the problem (e.g. sometimes two or more programs running at the same time may conflict).
Also, how much RAM does your laptop have? I think you should only consider running Vista if you have 2GB or more of RAM, Vista is kinda memory hungry.
I hope these suggestions help.