1. Check that the new physical memory as been recognised. It should be, but let's make sure.
NOT increase virtual memory. Buy more physical; it should be dirt-cheap. Disable VM
if at all possible.
3. Get rid of the usual spyware &c.
4. Disable (or set to manual) any unnecessary services.
5. Clean the junk out of hklm/software/microsoft/windows/currentversion/run (and also hkcu).
6. Clean the junk out of the respective Startup folders.
7. Nail down your AV
. You don't need continual checks of the whole fs if you're already checking everything that goes in and out, surely?
And so on...
7. Check windowsupdate. It won't help your performance, but I always recommend it anyway
After doing all that, try looking at the process list, after a clean restart. How many processes, and how much memory in use?