Check the clock
Web applications depend on accurate time keeping. Sessions are managed using a cookie, and the server sets a lifetime for that cookie based on what it thinks is the current time. The client then receives the cookie and acts on it based on its own clock. Additionally, the server will direct the client to store responses that it sends in a cache until a specified time.
For these reasons, it is important that both the server and the client have accurate system clocks. In many situations, it is not possible to directly verify or control the system clock on the client, but it is something to be aware when trying to determine the cause of unexpected behavior. On your server, you can use a utility to automatically synchronize your clock with an authoritative time source.
Most versions of Windows have the ability to synchronize the clock using an Internet time source, but third-party utilities are typically more reliable. The NTP Pool Project is a reliable time source that is available for general use; see http://www.pool.ntp.org/en/use.html for setup instructions. Another useful time utility is at http://www.worldtimeserver.com/atomic-clock/