Interesting, and welcome to the site.
If you go to Help > about in WMP10, what version number do you see listed?
There have been two versions of WMP10, the current one I believe to be 10.00.00.3802. If yours is lower than this, please re-download the installer from
http://www.microsoft.com/windowsmedia/
Now, as to the specific problem. It sounds like you are trying to upload protected content to the H/PC, and the HPC is refusing it as it cannot sync the security key. Have you been buying music from MSN Music, Napster etc -or- been ripping music using WMP itself?
If you're using WMP itself, head into options, the RIP tab and remove the Copy Protect Music check box. OK that and try and rip some more music from a CD. If you then try and upload the newly ripped music, it should go through correctly.