HP Pavilion xt963 puts monitor in "sleep mode"

Hello! I have an HP Pavilion xt963 tower that was working fine the other day but when I went to boot it up it said there was a file missing and wouldn't start up. It runs Windows XP, SP3. I tried safe mode but still no avail. I tried to boot up from the CD to reinstall the file, but it requested a BIOS password, which I didn't have. I then read that if you take out sticks of RAM one at a time and try to reboot it, that would work; however, when I took the first one out, it beeps a couple times when it boots up but nothing comes on the monitor...the monitor actually shuts off in sleep mode. When I unplug the monitor from the back, the monitor screen comes on, but as soon as I plug it into the computer, it shuts off. Any help is appreciated!