sounds like your user profile is corrupt.

if you have multiple accounts, the first thing you are going to want to do is login as another user.

if you do not have multiple accounts, turn off the computer then turn it back on. before the windows loading screen comes up (the screen with the scrolling green bar) continuously hit F8. a menu should appear. if the loading screen comes up, you have not hit F8 soon enough and must reboot and try again. in the menu that appeared, you should then click the administrator account icon

now either way you should be logged in.

now go to my computer > c drive > documents and settings > the user account of the account that will not login and copy its my documents folder and favorites folder somewhere else, as you will want to have these when you create a new account.

now go to start > control panel > user accounts and select the account that will not open

and select delete account.

now reboot, login as the second user again, once again go to control panel > user accounts and recreate that account

log into that account then log back out and back onto the account you used to create the account

now go to my computer > c drive > documents and settings > new account name

and move the documents and favorites folder that you copied earlier back to the new location. you may be asked to overwrite files. this is fine.

once you have done this, reboot and you should be able to login. if you cannot log in now, then you have a problem with startup items and i will need more information from you.