sfc.exe prompts for winxp cd ?

i just built a new P4 system and installed WinXP Pro

when i installed WinXP Pro i set up my drives as

DVD-rom - primary ide master
CD-RW - primary ide slave

but later after winxp was installed I changed it to

DVD-rom - primary ide master
CD-RW - secondary ide master

now when i run sfc.exe it prompts me for my winxp pro cd and says system files need to be copied to dll_cache for it to run but when i put in the winxp cd into either of the dvd-rom or cd-rw drives it still doesn't recognise winxp cd

it says either the cd-rom drive isn't working or wrong winxp cd ?

i know the drives are working cause it can run winxp cd's setup and boot cd as boot up fine

the only thing i have done lately is flashed the dvd-rom to a region free firmware

any ideas ?