please insert the cd!? its in there! help please!

[nockienoo]

hi there. thief 3 installs fine but when i come to load the game i get the load up screen from pc gamer (budget version) and the a delay of about 30 secs then a pop up appears with 'please insert the theif 3 game disk into dvd-rom drive' obviously the dvd is alredy in the dvd drive! after i click ok, if i go into my computer the dvd drive has now dissapeared! i have to restart windows for the dvd drive to appear in my computer again. it also disapears from devices in control panel...wierd! i really wanna play the game can u help? my system is:

amd athalon 3500+ 2.2 gig processor
1 gig ram
ati radeon x1800xt graphics card (bought specially for theif 3)
windows xp up to date ati drivers and the theif 3 patch from this site
the dvd-rom is a 16 speed
thanx every1