Ati radeon grafix card 9550/x1050 monitor signal out of range

i have a ati radeon grafix card 9550/x1050 have my monitor pluged into the vga port and tv pluged into s video port the problem i have is when i turn on pc during boot up i get a message on the monitor signal out of range. the tv however is fine.when the os has loaded displays are both fine. if i unplug s video lead the monitor displays boot up.but they both used to work together. i have recently put windows7 on a seperate drive dual boot with xp on the other drive. i am prity sure i had this problem on the morning of this new settup. i have also tried updating drivers for the grafix card.