Boot the machine into "Safe Mode with Command Prompt" and enter these commands

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net user NewAdmin /add
net localgroup administrators NewAdmin /add
shutdown -r
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This is an explanation of what the commands do:
1/ This will add a new account named NewAdmin onto the machines profile list, with no password required.
2/ Add the NewAdmin profile into the local machine administrator group.
3/ Restart the machine to exit safemode and bring up a normal Windows session.

After the reboot you can then login with the NewAdmin username (it has no password) and you will have full admin rights to the machine.
You can use this temporary admin account to create a new user account and copy all of the MyDocs etc over for the new user.

Once you have logged in with the Repaired user account, remember to delete the NewAdmin account or at least add a password to keep that account secured.
You don't want an unprotected admin account to have full access to all of your files!

Regards

Alun