When Vista starts I get the login screen. I type my password (it is an admin account). Then a message "Welcome" is displayed in the center of the screen with the rotating vista progress cursor to the left. But then I get an error message that ‘Access is denied’ with a red cross underneath it. This is in Normal mode. When I try to do the same thing on Safe mode, I can log in just fine which makes me think that it is not a password-related issue. So I’ve created a new (second) admin account and I can log into this one ok when in Normal mode. But when I try to switch to the original admin account (and subsequently denied) then try to return to the second admin account, the same ‘Access is denied’ error comes up (even though I could log in previously). After restarting the computer, the second admin account is fine again. Has anyone had this problem before, and what can you suggest I can do to fix this problem?

If it works in safe mode, then there is a program in the startup that's blocking it. Run msconfig, go to the startup tab, uncheck what you don't need to load at startup, which is probably most of it, reboot, click ok and try it again. Stay in selective mode.