Log on to the Computer

Log on to the computer using the Administrator (or an Administrator-level) account.

Crawl through the Registry

Open the Start menu. In the Start Search area, type regedit and press Enter (if prompted by UAC, click Continue/Yes). In regedit, go to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList

Expand the ProfileList key and look for the SID key (named "S-1-5"...) with a long number that ends in ".bak". Click it, and look at the ProfileImagePath value in the right pane to verify that this is the user account profile that has the error.

a) If you have two SID keys with the same number (with one ending in ".bak" and one showing the affected user account in the ProfileImagePath value), continue to Step 3;
b) If you have just one SID key with the ProfileImagePath value showing the affected user account, proceed to Step 4.

Two SID keys with the same number

a) Of the two SID keys that corresponds to the affected user account, right-click the SID key that does NOT end in ".bak" and click Rename.
b) Add .bk to the end of the numbers and press Enter.
c) Right-click the other SID key that DOES end in ".bak" and click Rename.
d) Remove only .bak from the end of the numbers (so that it has the same name as the other SID key did before you renamed it) and press Enter.
e) Now go back and Rename the first one with .bk to .bak now at the end of the numbers and press Enter.

Proceed to Step 5...

Only one SID key ending in ".bak"

a) Right-click the SID key that corresponds to the affected user account and click Rename.
b) Remove only .bak from the end of the numbers, and press Enter.
c) In the right pane, right-click the RefCount value (if none exists, right-click the right pane and click New and DWORD (32 bit) Value, then type RefCount and press Enter), and click Modify.
d) Type 0 into the Value Data textbox and click OK.
e) In the right pane, right-click the State value and click Modify.
f) Type 0 into the Value Data textbox and click OK.

Try logging on again!

Close regedit and restart the computer. You should be able to logon now...