HELP: My Dell is as slow as a snail

I am hoping someone can provide me with some advice on this issue because my Dell is so painfully slow that it is a misery to use it at this point. I'm not a techie so I don't know where to look.

I own a Dell Studio XPS 9100 working with Windows XP.  I have a broadband cable modem with a brand new router. 

The system is unbelievably slow.  On top of that, at least 50% of the time I load a page on the internet, I get a message that the system is "not responding" and I lose connectivity.  Even when I connect, pages take forever to load and I almost never get to watch a video clip without it stopping in the middle. I tested the following potential problems:

1)  The Browser:  People always say the problem is Windows Internet Explorer.  In order to address this I loaded Firefox and Google Chrome.  the same connectivity problems affect those browsers too so I don't think it is Explorer.

2) The Cable Connection:  I completed the Ping test and the speeds are within a healthy range.  But when using a laptop off of the same router, there are no connectivity problems.  That would seem to rule out the cable company, but perhaps there is still a problem with the connection to my PC?

3) The Computer:  I feel like there is something wrong with the computer, but when I do the Dell Diagnostics Test, everything comes up as "passed test".    It can't be hard drive storage because I have 90% free space

4) My Memory:  How do I know how much memory I have and whether it is sufficient for today's applications?  It may be that too many programs are running at the same time but I don't know how to check this.

5) My Virus Protection:  I have McAfee and BitDefender and each time I do a test there are no viruses.  That having been said, could it be that having two virus protection programs slows the system?  Should I get rid of one of them? I also have an Anti-Malware program that shows nothing out of ordinary.

Any help would be appreciated.