James the thing is with memory. Its what programs use to run their applications.
Therefore the more programs you have running the (each takes up some memory usage)
the less available memory you have.
Do you have any icons next to your clock?
If you do then these are programs running in the background, all using memory. Do
you need these programs running, all the time?
Virus checkers are normally memory hungry. Do you actually need that running all
the time. I have a virus checker but I only use it to check suspect files.
If you Run: msconfig this will show you a list of programs that run as soon
as Windows starts. You can enable/disable programs to see what difference it makes.