Acer Aspire 3000 Notebook - major memory problem!!

I tried to upgrade the memory in my laptop. There is a 256MB Samsung stick in there already, so i bought a 1GB Kingston stick to go in the other slot (Kingston and Samsung are supposed to be completely compatible). After he put it in, the guy booted it in the shop, and it worked fine. I took it home, booted it again, and it worked fine. I then shut it down, and tried to boot it again, when I had problems.

In the next 18 hours, there were 12 unprovoked restarts (for apparently no reason), 3 'serious errors' in windows, 1 'serious error' in my broadband internet connection, Nortons and MSN shut down. It then told me to reinstall Nortons, and Windows Explorer (I have XP).

Each time I rebooted, it found something different wrong, before just restarting itself.

It also said something about my monitor having an 'Invalid Licence ID'.