windows 32 bit only supports up to 4GB (updates wont change this) but you actually get about 3.27GB available, some people get 3.5GB. depends on the system. if you have the 64 bit version of windows xp it can use the whole amount.
when you get them you only need to plug them in. while the system is off (as in unplugged). plug them in snugly and make sure the retaining clips on either side are holding the sticks of ram. boot and your computer should show them in the post screen, or more appropriately the size of the total ram in MB possibly KB if your mobo is old enough.