Monitor "Out of Range" Message

Hello,
Please help,
I installed The Sims 2 but the resolution was too big, even if I changed it. So I went online to find out a way to sort it out. Many people were saying to go to the desktop, right-click, properties;settings and change the resolution from there. I did this, and I was a bit confused by the 2 squares representing 2 monitors (?) so I changed the resolution on both. There were also 2 options below(I can't remember what they said) I realised they were changing something but it wasn't apparent at the time. I tried to sort it out to go back to normal, and I thought I did it but then the monitor went blank/black. The computer didn't restart, but remained running. Then an "No Input Signal" message appeared, however this is normal. Then an "Out of Range" message appeared, however, recently this has been normal too, it would appear for a few seconds then the computer would run as normal. This time, however, the message remains and moves around the screen.
I have tried restarting it and it does beep when it comes on (which I've heard is supposed to happen) but just says "No Input Signal" and "Out of Range" as before. I have also tried F8 but nothing happens and disconnecting and reconnecting the monitor but to no avail.
This is a Windows XP computer (atm I'm using a laptop). My dad recently repaired/replaced our old computer (which was also an XP) but with a slower and (for want of a phrase) less compatible comp so this is probably the reason the resolution for The Sims 2 wasn't right in the first place.

P.s. Please, please, please help my dad only recently repaired the computer because it kept turning on and off randomly and I know he would be angry for it to be "broken" so early on. I only have until Friday or OR HE WILL KILL ME. Also, it would be best if I didn't have to buy anything as I can't really go to that length, plus, neither really can my parents, money is tight :L.

Thanks in advance!