Can you do it in safe mode???
if yes then you need to run checkdisk, scan for infections, and/or create a new account and trasfer your data to the new account and delete the old owner account as you might have a corrupted account. see instructions below
http://support.microsoft.com/kb/315265/en-us
How to perform checkdisk
if you do not have Recovery Console installed, then run from the start menu
go to start, run, type in cmd
in cmd type in chkdsk c: /r and press enter
type Y for yes and press enter
type exit and press enter
restart pc, allow checkdisk to finish and run again
this is a lengthy process depending on the size of your harddrive, the percentages will fluctuate, this is normal, you can view report in the event viewer.
go to start>run> and type in eventvwr.msc, click on the applications directory, and in the right hand panel choose winlogon.
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and what is your current antivirus??
Download, install, update and do full scan with these free malware detection programs:
Malwarebytes (MBAM):
http://www.malwarebytes.org/products/malwarebytes_free
SUPERAntiSpyware: (SAS):
http://www.superantispyware.com/
Remove all found, reboot, and run again until scans run clean, these can be removed afterwards if you so choose. Run scans one at a time, please.