Put your installation/recovery cd into your disc drive and then go to Start-> Run (Windows key and R if it doesnt show up on start menu)
In the run box type "SFC.EXE /SCANNOW" Without the quotation marks, if you are on Vista, then click run as administrator.
This will run the System File Checker which attempts to replace any damaged system files.
If this manages to replace any system files then run Windows Update as soon as it is finished, if it doesnt help then try a REPAIR installation from your isntallation cd.