I have a Lenovo g560 3-4 years old, Intel i5, with 2GB ram and with a Geforce 310M. It always had some issues during startup or when starting a new program, but recently it just went crazy.
From the 2GB I always have 300-800 Mbs free, and the cached memory rarely goes above 300 (from 1.9gb maximum), the cpu performance is great (barely reaches 40 percent). Even like this, when I start or even just swap the screen to another bigger program the change of the windows becomes really slow and gradual (e.g. I see the 'pop-up' visual effect, but have to wait for the new windows, which appears only very gradually, first the edge of the frames, then other parts etc). At the same time the mouse cursor's movement is fine (but not the context-dependent changes), and I also got immediate response for the alt+tab combo (that is, the popup window with the opened applications). The lagging also stops sometimes when the program has already loaded in the window.
My suspicion would be that the hard drive causes the whole thing, because its led works a lot, and the laptop had always a considerable degree of lag when I had to manage files or start programs (especially compared to their 'loaded' speed). On the other hand, however, the resource manager does not really show too much disk usage (or I just do not see it)
I ran Winzip Utilites to optimize the computer (malware/junk search, disk defragmentation, driver updates, registry clean-up) but It made things even worse. I think the reason for this was that it turned on the Aero theme. When I changed it back to basic (after the Windows noticed the slowness and asked about that) it did some help, now it is 'just' slow.
Some resource monitor screenshot during lagging: