Cancel the login process in Windows

Just go to the Control Panel in the Start Menu, click on User Accounts, and there you can change that status. You can either turn off one of the users or simply disable them.. if you are still having trouble write another message and I will explain in further detail.

lior_p is right, "Change the way users log on and off of this computer".

If the answers above doesn't fit your problem, you can try another thing. but be careful, it demands you to touch the registry, and it's at your own risk.

run regedit by clicking Start > Run > regedit

then you should go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon

there modify/create the value data types and value names as detailed below:

Data Type: String Value // Value Name: DefaultUserName
Setting for Value Data: [Enter the account name to be automatically logged on]

Data Type: String Value // Value Name: DefaultPassword
Setting for Value Data: [Enter the account password]

Data Type: String Value // Value Name: AutoAdminLogon
Setting for Value Data: [0 = Disabled / 1 = Enabled]

If you don't know how to use the registry you can use TweakUI.