I have a file and print server running on Windows 2008 R2 fully patched in a VSphere environment (ESXi 5.1 fully updated). Every evening between 19:20 and 19:30 our monitoring software reported that the available memory is 1% and performance is dire. There is nothing in the event logs to point to an issue. At this point in the evening I am general the only user on the system to check to see why these alerts are going off. Things I have done; Checked to see if any backups are running – None at all. Checked Scheduled tasks – None before or during this time period. Moved the VM to another host. AV is disabled to rule out that as the issue. The server does not have any problems during the day with memory when fully loaded with about 50 users. The server did have 4GB ram provisioned but I have increased this to 5Gb. Running PrefMon at the time (I will save the graphs tonight) There very little CPU usage at the time but RAM usage goes up.