I just tried adding an extra 512 to my compaq presario desktop pc. I'm sure the Ram type is correct. Though when I check different sources it varies between pc2- 3200 and pc2- 4200. Some sources say both are compatable. I'm not sure what the difference is. Anyway I shut down the PC, unplug everything, insert the additional RAM, plug everything back in, start it up...then as far as I get is the initial compaq screen before windows xp boots. You know the screen where it asks if you'd like to enter bios or setup, etc. If I hit f1 soon enough it enters the setup and I can see that it shows 1024 mb of RAM (the old 512 + the new 512) but my keyboard won't work at that point so I can't do anything other than power back down. So it seems to be recognizing the RAM but why won't windows boot? Why won't my keyboard work after I enter setup? When I remove the new RAM everything works fine again. Any help would be much appreciated.