Disconnect form the internet,

turn antivirus or internet security off.

Run Disk Cleanup (start>all programs>accessories>system tools> or search>disk cleanup), select c drive, select the check box next to all entries except compressed old files unless you want to get rid of them also then ok.

Next run disk defragmenter (start>all programs>accessories>system tools or search>disk defragmenter) select C drive then defragment.

When defragmenter finishes Open MY Computer and right click the C drive, Select Properties, select the Tools tab, under Error Checking select Check Now, then select both check boxes then select Start, answer Yes to the Schedule to run at the next start up then restart the computer.

When this finishes it will reboot the machine then just log on to the user as you normally do then restart your internet security or antivirus program.