Sometimes it will come on and freeze in starting xp other times i just get black screen

GO GET A BATTERY PLUG IN A TRANSFORMER

same thing happens to me - most of the time it will just randomly freeze; occasionally i get an unresponsive black screen. this 24' aluminum imac usually has startup problems also - sometimes it will "gong" and nothing else; sometimes the apple logo will come up and then nothing else; sometimes it will load the logo and then spin for a few seconds and then freeze - the only option is to manually shut down. there is no rhyme or reason to it

That's a common problem with Vista...you'll need to restart your computer. Start back up in safe mode...by pressing F8 at the boot screen. then restart your computer normally through the start menu. After this I would find a good copy of Windows XP Professional and back up my data, and install XP over vista.

i am suspesioning on Rams. maybe overheating also does that.

remove the ram and blow air and clean it in case dust might be formed
fix it and clean the system once and check all socket fittings once and turn it on 

I suggest you try formatting the hdd and reloading the operating system as a first step and if that doesn't work there is probably an internal hardware problem.

only thing I can see is for you to re-install WIndows xp. seems like the operating system having a porblem loading.

start up windows in safe mode and see if you still get the problem. did you use the same vga cable???

It seems like you had a software conflict...

basing on the information you have given here are the possible solutions:
remove any external device connected to your system (printer, external hard drive and others) if removing this does not work and still gets a blank screen with blinking cursor try to go to safe mode with networking. if successful try to download the driver for your video adapter.

when you try to start the computer , do get anything on the screen like a logo? when does the blank screen occurs, is it during the turning ON of the computer or during the windows xp logo appears on the screen?

a blank screen could be caused by the following: defective video adapter, corrupted video driver, defective memory, unknown device causing the issue like the hard drive, optical drive or other peripheral installed inside the computer.

try also using an external monitor