I couldn't fix the problem with restarting my computer or uninstalling/reinstalling Steam. When I launch Steam I see steam.exe and steamservice.exe in the Details tab of Task Manager, but the UAC virtualization column says the former is Disabled and the latter is Not allowed. steamwebhelper.exe periodically pops up in the Details tab, but immediately disappears after it appears. steamwebhelper.exe does appear as a Process in the Resource Monitor but is greyed out and says Terminated. Also, running Steam in Win8 compatibility mode doesn't help.

Any ideas?