Matched pairs are preferable, but not crucial (no pun intended).

The link referred to in the posting above actually explains this:-

"DDR2 memory modules should be installed in pairs of matched memory size, speed, and technology. If the DDR2 memory modules are not installed in matched pairs, the computer will continue to operate, but with a slight reduction in performance. See the label on the module to determine the module's capacity."

I wonder if Kasperspy is running some sort of scan at startup that is causing the 'slow' problem.

The relevant figures from Task Manager are:

Physical Memory Total = the amount of physical memory recognised (from what you've said, seems the 3GB is recognised OK) Available = free physical memory. Seems like you are not using that much and maybe you didn't need to upgrade anyhow.

Under Commit Charge heading: Total = your current physical + swap space memory use Peak = your highest (since boot) physical + swap space memory use

It is this last (Peak) one that is of interest, as if the value is below the Physical Total (or rarely goes much above it), then you aren't swapping. And is it stays below around 1048000 (roughly what you had before the upgrade), then the additional memory isn't really adding anything.