I bought a second hand Compaq Armada M700 laptop.?

it seemed to work ok until i tried to get on the internet with my USB modem. After I installed Firefox and Antivirus it has slowed down so much and keeps trying to install Microsoft updated which never get completed. I have turned updates off to speed it up but to no avail. Is this because it only has a 64MB RAM? Do I need to buy a new laptop or can I fix this? I dont know a lot about computers so couldnt really do much technical stuff!