password at startup

I'm occasionally having problems entering my passwords at the startup
page (where the users are listed, you click on the username and are
prompted for the password). Some times when I click on a username, I
get the perpetual finger pointer and I'm not able to enter a password.
Whenever I click a different user name, same result. I have had to
restart the computer. This works fine, but I'd rather not have to do
this every time I need to log on. I run the disc cleanup tool and
defrag fairly often (at least weekly).

Thanks!