Slow Machine Not Much Faster After New Memory

I have a Dell Dimension 9100 with 3GHz Pentium 512MB ram, running windows XP. It was top-of-the-line when I bought it, but is now about 4 years old. Over time, the machine has been running slower and slower, especially since I run Apple's itunes, and firefox, which I assume are resource hogs from looking at the Task Manager's processes tab. Lately, it has taken forever for the machine to load windows, or wake up from sleep. Nothing dramatic or noticeable that I could point to, just a gradual slowing down. I always attributed it to an aging computer and lack of memory. So this week, I purchased 2 GB of additional RAM. I've correctly installed it and confirmed, from MyComputer System Properties, that I now have 2.5 GB of RAM installed. I was expecting a big change in speed, but there doesn't seem to be that much of a difference. I have Symantec FireWall and AntiVirus running, ( it was original to the system) and am afraid to remove it.About a year ago, under advice of an IT friend, I also installed AVG and Spybot Search&Destroy.