There is probably a way of protecting it with one of the free anti-virus applications such as Avast. The McAfee software comes minimally with at least antivirus, firewall and the optional browser add-on Siteadvisor.
Minimum possible RAM requirement is 256mb but more is highly recommended as it all depends what is already installed on the machine that may be utilizing some of that RAM & nowadays just about every application demands its share..
From what I read online the memory slots on the D600 are easily accessed underneath and can accept up to 2gb. I may be wrong of course, but I do highly recommend that you add more RAM. It is by far the fastest & easiest way to improve performance.
What operating system and service pack is installed? You can check that by right-clicking My Computer and selecting Properties.
moved to (hopefully) the right group for more help.