standard user accounts stopped working

My standard user accounts worked fine until recently. Now, when my children
log in, they get a blank screen -- no desktop, no icons, no cursor, nothing.
I changed their accounts to administrator accounts, and the accounts work
again -- although, of course, my kids can change things that I don't want
them to change. Anybody else have this problem? Did one of those relentless
Windows updates mess up the user accounts? How can I get the Standard User
accounts working again?