cdrw/dvd combo wont play dvds

Hi, i have constructed a basic pc for my daughter who has just gone to uni, i hoped with it she shall be able to use the web and watch dvds , anyhow its made up of older parts, with a duron 900 and 100 mhz sdram! still no problem getting on the web, the problem is with the combo drive, its a cdrw and a dvd player, its a bit strange,it has no problems with cdr media reads and writes ok, i have had it playing dvds ok, but once the pc has been turned off and restarted it will no longer recognise that there is a dvd in the drive, it actually believes there is no disk at all present, i have found that if i go into the cmos set up at startup and use the high performance default settings save and then boot, the drive will play the dvds no problem, but once the pc has been turned of the next time round it will again not recognise dvds, basically anytime i want to play a dvd i have to go into setup at the startup set the high performance defaults and it will work but its not an ideal solution, i have tried uninstalling the drivers and then letting xp load them back on, no luck there, i am using power dvd 4.0 as my default player, windows media player doesnt recognise the disks either, i do not have any burning software installed such as nero, there are two hard drives installed one of 15 and one of 10, gig which work fine,the drive is an asus cdrw\dvd combo i would be gratefull for any suggestions.