It is quite possible your computer does not have enough of it! Virtually all computers that came with Windows XP or Vista are 32 bit systems. 32 bit systems cannot “see” more than about 3.25 gigabytes of memory. 64 bit systems can see much more. It is a truism in computers that increasing memory (up to some reasonable limit) is the most productive upgrade you can give a computer. If you want to know how much is installed in yours, right-click (or left if your mouse is set that way) on Computer and choose Properties. Many computers have only 1 or 2 gigabytes. That is fine for XP, but you want 3 for Vista 32 bit versions and at least 4, preferably 8 for Vista or Windows 7 64 bit versions. For most uses, more than 8 gigabytes for a 64 bit system is not useful.
There are many different types of memory, so you must find out what your system requires and how many slots there are for it to be installed in. In some cases you can install faster memory than what originally came with your computer. However, performance will be no greater than the slowest memory component. Pay attention to the specifications of your main board. They specify the CL rating of the memory they prefer. In some cases, if you do not provide the correct CL rated memory, the memory will not operate in dual mode which will cause performance to suffer. Also pay attention to voltage specs. Commonly 1.35 or 1.5v.
In most systems, it is best to install pairs of duplicates.
Computer manufacturers charge huge premiums for more memory. Buy your new computer with minimum memory. Then buy (not at over-priced xxxxBuy) more and add it yourself. It is surprisingly easy. The web is full of instruction videos. Notebook systems have 2 slots for memory (called SIMMs). Desktop systems have from 2 to 4. Memory boards (called DIMMs) are twice as long for desktops. Most systems will take more memory than their manufacturers show on their specification sheets.
Installing memory is not difficult. Remove any power source, including the battery in a notebook. Hold the ON button in for at least 5 seconds. Notebooks have a removable hatch on the bottom. Desktops are even simpler. Remove the side of the case. You will see the slots. There are two spring-loaded clips. Open them. Line up the memory add-in board in the slot paying close attention to the slot on the board. Press the board in hard and you will hear it click.
Memory prices have a curious curve over time. When your computer is new, memory prices tend to be high. About 2 years later, they fall quite dramatically. Then after about 5 years they rise quite dramatically. So, take advantage of this and add memory at about 2 years of age.