I installed more memory but Windows doesn’t recognize it

Bad memory modules aren't very common, but test the computer with a different module if possible to see if the one you originally installed was a dud. The most common cause of this problem is that some motherboards require that memory modules be installed in pairs. Most RIMM memory modules, for example, must be installed in pairs that match exactly in speed and capacity or they won’t work. Newer systems that support dual-channel DDR SDRAM (Double Data Rate Synchronous Dynamic Random Access Memory) modules will work when the memory modules are installed singly, but performance is enhanced (sometimes dramatically) if you install matched pairs of RAM modules instead (i.e. you get better performance using two 512MB modules than from using a single 1GB module, even though the overall memory capacity remains the same). Check the manual carefully when installing memory to make sure you place the modules in the correct slots.