Few weeks back I had a strange problem, my Vista installation kept saying it is going to expire in 11 days. If you see the above image of the Control Panel-Systems says Windows is Activated. So I was puzzled, how can something that is activated can expire. Strangely the same error kept coming in few other machines in our office.
After several hours with Microsoft PSS on phone, they diagnosed the issue in my machine to be a time-bombed SP1. Since I was in the beta program of SP1, I had downloaded the RTM of SP1 from Connect which was time-bombed. The safe way is to get SP1 through Windows Update or Microsoft Downloads.
I uninstalled this SP1, installed a fresh one from download.microsoft.com; now my machine is fine. I had trouble with installing Windows Search 4.0, which also got resolved after the fresh SP1 install.