My system has 4GB of RAM installed but the OS shows only 989MB usable and the PC is very slow. How can I resolve this?
What does System Properties show as the amount of RAM in your computer? (Shortcuts: Press the Windows key + Pause/Break; or click Start -> Computer -> right-click and select Properties.)
If your computer has 4GB of RAM, the line "Installed Memory" should show at least 3.25GB, depending on if you have Windows 32-bit installed or have a graphics card that's built into the motherboard (not a card in a slot). If it shows less than 3GB, then at least one DIMM in your computer has gone bad or is not properly seated (locked in its slot).
I would highly recommend against cleaning the registry unless you know exactly what you are doing; if you are not careful and mess up the wrong registry key, your system can wind up unusable and unable to start after reboot.
I would suggest reseating the memory modules (take them out and place them back in tightly but carefully), try reseating the processor (check to see if there are any bent pins first), check the BIOS settings for memory options, make sure that the wiring inside the pc is plugged in properly and tightly, etc. Worst case scenario would be a bad motherboard that needs replacing.
You might have not cleaned your PC's registry. When you uninstall a software it lefts some of its registry behind which makes our system slow. One more reason can be that you might have not cleaned garbage data from your PC.
[Ed: removed link due to poor WOT rating]
How much total RAM is the computer seeing? If the OS can see the entire address space, your problem is with something that is being loaded and sucking up all of that space. Otherwise, you have at least 1 bad DIMM/SODIMM.
maybe you have to clean your computer (viruses, malwers) ...
The fastest (and expensive) solution would be to replace all memory cards since it's highly likely one or more failed. A cheaper but time consuming approach would be to remove cards one at a time. Once it won't boot, you've located the "good" one. replace the others.
which windows version you have?
open msconfig, then go to boot-advanced options, tick maximum memory and enter 4096.
There are all sorts of possible scenarios here. Where does the figure of available memory come from? Is it from the system properties window or somewhere else? Also, if you know how to get into the BIOS, can you do so and see if the PC detects the full 4GB?
Err, yes what?