Solved - Toshiba Satellite Pro L300 SOUND
Just thought I'd post on here as I couldn't find any solution while searching online earlier...
If anyone downgrades a Toshiba Satellite Pro L300 to xp, you won't be able to install the sound driver until you manually install the HDMI sound driver to the system device > pci device. (right click, update driver, from specific location, \Sound Driver\HDMI\XP2K)
(you can get the driver off the tosh website)
The pc will then detect your sound card and you can install as usual.