Have you added memory before ??
Install your new RAM
1.
Turn off the computer and touch the metal casing.
2.
Locate the RAM modules. Find the empty one you plan to replace, or remove the RAM module you will replace.
3.
Line up the notches of the new RAM module and apply firm pressure to attach.
4.
Once you're sure the RAM module is snugly in place, close the latch at either end. If you have clips, they should snap back in place.
5.
Reconnect all the cables, but leave the casing open until you're sure everything is working right.
6.
Turn your computer back on. If the machine starts to beep, the memory is either incompatible or not in correctly, If you've installed everything correctly, the system will detect the new RAM.
7.
Check the system properties to see how much RAM you now have. If you replaced a 128 MB with a 512 MB module, then you should have 512 minus 128 equals 384 MB more RAM than you did previously. If you added the RAM but didn't remove any, then you'd have 512 MB more RAM.
8.
Try one of your programs that wasn't working up to speed. If it still isn't working, unplug everything again and get back into the computer to check that the RAM modules are firmly secured.
http://www.microsoft.com/athome/moredone/addmemory.mspx
Some info for you