Be aware that your "Security Suite" software may be slowing your 'puter down considerably.
Some Anti* software can end up taking time and resources to scan everything you open locally as well as everything you open on the internet.
In addition to this, many of those programs do not give you much control over this except for you taking the time to designate certain files as "trusted" and certain files as "not trusted".
My Norton Security Suite will go into a full "idle time" scan when I take a two minute break in the middle of my working on something and it's a major convoluted pain to stop it.
Take a look at the programs that are automatically started in the boot sequence. You may not be using some or most of them often enough to warrant running in the background 24/7. Take the ones not normally used out of the start-up sequence. Many times it's your demand vs your memory resource that's slowing you down and not your HD.
While I certainly agree with kicken about the frag, I'm not sure if I agree about the amount of material on the HD not affecting response time.
From personal experience, every machine I've ever owned, going all the way back to ms-dos, was 5 to 10 times faster when it was brand new than when it was years old and 3/4 full.
Before anyone says anything, yes, that's with cleaning up files and registry and de-fraging every two or three days.
You've got to keep in mind that the more material that gets read, modified and re-written, the more fragmented your HD is going to be. The computer is not only collecting parts of files from all over the HD, it's also working hard searching for small areas of free space to write packets of data to.
What really blew me away was that I spent $30 to upgrade my old desktop memory from 1G to 4G (3.3G useable) two days ago. Now it doesn't matter if Norton does a full scan while I'm still working or not.
I recommend this above all esle. Memory has gotten dirt cheap as I thought it would. Find out what your machine's full memory upgrade limit is and spend the measley few bucks. Do it even if it's a brand new machine. Immediately. Pass me some reputation when the thing throws you back in your chair after you do the upgrade.