Shuts down ok then restarts

I am running windows xp on a gateway computer and have been running into two problems.

The first one being that when I shut down the computer it does so just fine but after a few minutes it restarts itself. This was never a problem before but have noticed it has hapened everytime I shut down after I installed an ether net card.
I have done a system restore and checked for bad sound files and all the other things listed in the troubleshooters and still no luck.

Now my other problem is that Internet Explorer has an error message that comes up stating it had an error and needs to shut down. I can ignore the message and just stay online but I have no idea why it keeps coming up. When I hit the details button it just has a bunch of file names with zeros.

If I do a system restore it wont do this for several days but pops up again later.

Any ideas??

thanks