Have a quick look here,
and see if there is a folder there that looks like this,
if your Windows is 32 bit (you didn't specify 32 bit or 64 bit), and this is 64 bit,
If it isn't there then it is really gone (System Restore would have fixed it in a few minutes). You can try adding that folder back using this one if 32 bit, cdrom.inf_x86_neutral_db87d184bc84f910.zip
if it is 32 bit, and then see if it will install. If it is 64 bit, unfortunately you'll have to wait for someone without the SP! beta installed (unless you are running the beta), as the files have been updated.