before you continue .... did you run the vista upgrade advisor?
http://www.microsoft.com/windows/products/wind owsvista/buyorupgrade/upgradeadvisor.mspx
just like when XP came out ... many older machines weren't suitable.
Vista uses a different type of codebase ... so norton, zone alarm etc etc etc (even office 2003) - may have compatibility problems ....
to make things worse manufacturers such as acer have delayed vista drivers ...
so all in all upgrades are a risky business
run the advisor BEFORE upgrading ... and you'll have a much smoother ride
Avoiding this simple step ... results in the replies you have here