Just to double check
the registry settings
Enable or Disable the CD-ROM Autorun Feature

You can disable the CD-ROM autorun by changing this setting, this will stop applications from auto launching when you insert a CD-ROM disc into your drive.

Open: trolSet\Services\CDRomHKEY_LOCAL_MACHINE\SYSTEM\CurrentCon

Change to value of 'Autorun', or create a new DWORD value if it doesn't exist: (0=disable, 1=enable)

Restart Your Computer.
are you restarting afterwards?

Try this patch if it does not work
http://www.aumha.org/downloads/autrun1.zip

You can also right click the device, choose properties, autoplay, and then manually configure the autoplay feature to work with a specific program....
What kind of CD do you want to autorun? Music, or soft?

Other than that,
try to run a sfc /scannow this will repair any damaged system files, but make sure you have your windows XP disc available as if there are any damaged files it will prompt you to insert your XP cd.

Try a system restore, to see if this will correct the situation,

other than that...
reformat and start over, as I have ran out of suggestions.
BB