Dell 9020 problems

Computers are all Dell 9020's with Win 7 Pro 64 bit. Drivers and bios are all up to date. Custom image. The problem is that when the users try to logon in the morning the computers will freeze either after clicking OK on our AUP warning which is before the logon screen or they are are able to logon but then the computer freezes. The only solution is to hold the power button down to do a hard shutdown. After the computer is restarted, everything is fine until the next morning. rebooting the computer multiple times will not replicate the problem. The event logs show nothing that would relate to this issue. This issue started after MS patches were applied last month. This is happening to at least 100 computers across campus, with about 50 in my department at several locations. I would like to know if anyone else has seen this and what the solution, if any, there is. The only thing we run at night is MS Security Essentials full scan but the event logs show it completing successfully and other normal events are logged after. Thank you Reddit!