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Dear Shekhar,
I had same problem. I tried to restore to earlier restore point but there was no restore point available. I had tried to boot in safe mode but same result stuck at welcome screen. this is same for all accounts. I tried to use installation DVD boot but it opens repair window tried almost all option but no result. After menory checks option i got one error "LogonUI.exe - Application Error" Further message is The instruction at 0x73DFB4D2 referenced memory at 0x73DFB4D2. The required data was not placed in to memorybecause of an I/O error status of 0xc0000185. Click ok to terminate the program.
Please help..