Dear yafim , r u fully sure that your computer is infected by virii/adware/trogens/etc.....then do the first step and if u r not sure and think that your window files are damaged then go for the second.......even if these don't solve your problem then plz make me aware of it..
1) Down load MICROSOFT ANTISPYWARE
>>>just run in for a scan.....and i m sure if you have any of the spywares etc they will be caught redhanded.
2.0)go the following page >> Click here
>>this page will definately remove any missing file from your computer / OS
2.1)Even easier process is to use sfc
the procedure is given on following page SFC SOLUTION
Even then your problem is not solved >>
plz make a reply as their can be dozens of minor problems....i have suggested for major problem that u could face