I have a compaq presario desktop machine, which I upgraded with a 512MB DDR RAM. The problem, which I'm facing is that at times the keyboard as well as the mouse stops functioning for a while and this has become quiet frequent.
I approached few of my IT friends with this regard to which they said that uninstalling some unwanted applications might bring back the system to normal, few of them advised me to restore the OS at an earlier restore point & some say that re-boot the system with setting it to the last known good configuration that worked.
I have tried all this but was always in vain & couldn't understand why this has to occur.
The possibility that I have in mind is that, my System's existing RAM was 128MB DDR RAM which I think processes data at a frequency @266MHz & the additional one which I have installed processes data at a frequency @400MHz.
So, I doubt that due to this processing speed difference there might be some kind of a data traffic that could choke the CPU to some extent, but then I am not sure as to whether this could be the cause.
So, I request you to please give me some more solutions to this as I think this can get more worse if ignored...

No, RAM is backward compatible. It will just run at the lowest speed

e.g if you use PC133 and you put in some PC1024 it will work, just at the slower speed.