Correlation between number of programs installed and performance ?

Hi,
I was having a lot of performance problems on my computer. I have a DELL
desktop with a 2GHz Intel Pentium, with 512MB of RAM and an 80 GB hard drive.

Previously I had a 40GB hard drive on which my computer was showing really
bad performance. When it went bust, I bought a new hard drive and installed
it in and took the old one out. I found that this replacement of the new hard
drive really bumped up the speed. Firefox, for instance came up with a much
shorter delay. An interesting thing I noticed is with the old hard drive,
Firefox wasnt visible on the screen until it had got about 38MB of RAM space.
Now with the new hard drive I was seeing it come up with as little as 15 MB.

However from then on I installed several other programs on my new hard
drive. The Norton Antivirus, s/w from the web etc. etc. And what I now see is
that firefox is back to its old ways in that it takes just as long to start
up. Looking at the task manager I see it again does not show until it gets
about 38MB of RAM. The computer too is now as slow as it used to be.

Is there anything that can be done here ? Is there a correlation between the
numebr of applications installed on a computer and the amout of RAM a process
needs to properly start up ?