Dimension e520 numbers light up on boot up but turn off after booted up.

I just replace power supply recently and computer was working good. Then two days ago was working on emails walked away for five minutes came back sat down at computer and got shocked when I touched my mouse and a moment later computer gave message that usb device was not recognized and after several reboots and it worked again.

Then today all was fine except that said key board error, or failure then worked fine for a while then out of blue it decided again to say that keyboard failed and this time could not get it to come up and did not have any codes or beeps or lights come on but when installing new key board and mouse which are working using as I write this but on reboot after installation get lights 1,2  AND 3 On and had to reset clock for cmos. but then also last reboot got a symbol that looks like cylinder or battery lighting up and the cmos battery was just replaced when power supply was replaced, also the lights flash for a bit then are solid but then seem to go off... how serious is this and how long will it last before it crashes computer and also what do these mean?

all input appreciated