First Try a Microsoft System Restore, Boot from a Windows 7 install disc and select "repair your computer", then select "System Restore", pick a date before you deleted the folders, see link below on how to boot from the W7 disc.
If this does not work move to the next solution
Use a W7 install disc and boot from it to get the recovery environment, then use the command prompt to attempt to enable the hidden admin account,
hit enter after each these 2 commands.
net user administrator /active:yes
How to get to the command prompt from a W7 disc.
This is for Vista but is the same using a W7 disc.
After you enable the admin account restart and see if you get a new admin icon, see if you can log into that account, if you can create a new user account in control panel for another way to log in. You don't want to use the new admin account for everyday use. If you get it repaired you can disable that account by using the following command while in Windows
net user administrator /active:no
If you still get the error and you did delete the C:\users\default folder, you will have to get a copy of that folder from another windows 7 PC and copy it back where it belongs, use an Ubuntu boot CD to do this, the link below is for backing up data, but it could be used to copy a folder to the hard drive also.
**NOTE: you need to use a regular Windows 7 install dvd, a "recovery" DVD from some PC manufacturers (HP etc.) will not work, if you do not have one you can download the proper ISO file from this link, then burn it to DVD as an Image (not data). Be sure to pick the proper version (Home,Pro etc) and bit (32 or 64)