black screen

Hi ppl, I have searched your posts & read 500 of them for my problem. My moniter will not get a signal half the time & the other half when i pull out vid card & leads & replace, the comp will power up but go black when it trys to load windows xp. It will not load in safe mode or any other modes for that matter-)just goes black, have tried 2 vid cards with same problem. Sorry if i cant give to many details on specs, it is a Athalon XP1800, AMD R7 cpu the v/c i was using was a radeon 9600xt 128 ddr. Also just tried a winfast a170. The monitor seems to reconize the winfast at least? but still cannot boot in any modes. It has been happining for some time now but if i persist with starting & restarting it will normally come good, but now i cant get it to boot at all, have tried in vga mode with no vid cards installed, same problem...
Sounds Serious Any ideas would be great thanks

 

reset cmos, also try new memory.

 

ok ta, can you tell me how to reset cmos

 

Well pulled battery out & back in, now message: is cmos battery low,
cmos memory size wrong. f1 for setup, f2 for default. Have tried both & starts to run through drivers ect. then goes black? I would think a new battery is in order. Tho the memory size baffles me, any thoughts?