I think what they mean is if you can boot to the live CD, use programs, open files, browse the internet, without problems it's probably a software (windows) problem. If not, if programs in Ubuntu lag still and you have the same or similar application problems, it most likely is a hardware problem, RAM, CPU, H/D etc.
edit: Have you downloaded and run memtest yet? If not, it would be a good tool to use. I am starting to suspect RAM.
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If your equation requires either a complex combination of variables to get the desired result OR you can't seem to get the result regardless of the variables....it ain't your variables, it's your constant.