There are a number of reasons besides the OS, Worms, Trojans, Parasites that are not detected by Norton (it doesn't consider Web Hijacking a crime!) and then you have hardware issues.

All that is written in your post leads to the obvious that you have a defective product and need to make it their problem.

Or tell a lot more.

Here's my post about Pests, Parasites and Web Hijacking:

- Parasite removal programs.

http://reviews.cnet.com/5208-6132-0.html?forumID=32&threadID=1313&start=0 notes the five scanning tools and removal tools that are fairly safe. If one does encounter an issue, its always been damage done by the MALWARE. The most common issue being Layered Service Provider replacement which you can use LSPFIX (Hint: use http://www.google.com to find).

The thread has follow on discussions about new links if you don't want to use Google to find an item.

Bob

PS. All the tools are free.