Windows Media Player/ Installshield Question

3 days ago, I tried repairing my Windows installation using my XP disc (I have XP home). I was running service pack 2, and it downgraded me to service pack 1, and basically messed everything up. I was pretty sure I was missing some of the install files, which is why i used the CD to repair the installation.

Anyway, now I cannot download Service Pack 2 through Windows Update, nor can I reinstall WMP 10, it gives me the error "An internal application error has occured." when I try to start WMP.

I tried installing Itunes, and it gave me an error relating to installshield, which leads me to believe that installshield is missing key files.

Hopefully this explanation can lead to a solution? ALl of the microsoft support was extremely unhelpful, and my error didnt show up at all in the list they had. Thanks in advance!