Ok I am going to give you some basic things I look for when I am helping computer-illiterate friends.
What kind of internet connection are you using? Dial-up, Cable, DSL, what?
DSL Reports Speed Test This will give you an idea (its not completely accurate) of how fast your internet connection is, and latency.
What are the computer specs? RAM, Processor speed, GFX card (if there is one), etc.
All of those affect your computer's performance.
Are there malware which is basically adware, spyware, etc. Software that shouldn't be running in the background that are running in background. If you want a better analogy, a rat that keeps stealing your food constantly.
Lavasoft's Adware You can try that program, install it, update it, then run a full scan. Its not going to catch everything, but its going to catch some things. There are tons of programs out there, it just happens to be the one I use. If you have a virus scanner run that too, its unlikely thats a source of the problem though.
Don't even start looking in dxdiag yet, you don't really understand what should be there and what shouldn't.