2 problems with computer

im really sorry if this is posted somewhere else but ive been searchin on various forums and couldnt find the answer anywhere. I have two problems with my computer: i turn on my computer and if it even starts to run it may get as far as the welcome screen and then the monitor will shut down but i think the program still runs but it will either shut down by itself or i have to do it manually. this was my first problem awhile ago. second problem is when i turn on the computer it instantly switches on and off on and off causing a clicky sound.first time i thought it was terrible and turned it off as quickly as i could. now that im used to it ive left it for awhile it will end up booting to the welcome screen but then the monitor shuts down. Ive cleaned it well, ive tried switching the monitor (same problem). Ive had this computer a year and 1/2 and its only started doing this now. i got a video card installed back in January and its been fine up until now. im stuck ive tried reinstalling xp but still these errors come back a few days later....

It sounds like a bad power supply. I wouldn't try to use the PC too much until you fix the problem. Turning off and on like that can't be good for the components.