display problem??

[yellowsnow]

hi
first time here.
now my problem,,i turned my pc on and the screen came up with a message saying "signal out of range". I changed the monitor and now i cannot change the display settings,,when i try to the sreen just gets coloured flickering lines running down it and then it goes back to normal.
hope someone can help.
wondered whether it could be cos this monitor i borrowed is quite old.
thanks for any info i receive.
regards simon