you probably have an infection
1. what is your current antivirus?
2. what other scanners do you have installed?
3. run a scan with your antivirus until it comes clean
4. download and run the FREE malwarebytes antimalware , let it remove all and run again until scan is clean, reboot as necessary:
https://www.malwarebytes.org/antimalware/
5. run checkdisk, if you do not have the recovery console installed or do not have an updated cd (sp3 is slipstreamed into the install cd), then you can run from within windows:
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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let me know the results of all the scans.