Here is what you can do.
1. Download comdlg32.zip and save to Desktop: windowsxp.mvps.org/utils/Comdlg32.zip
2. Unzip the file using WinZip or any other utility.
3. Extract comdlg32.ocx to Windows\System32 folder.
4. Type the following command from Start, Run dialog: regsvr32 %Systemroot%\System32\comdlg32.ocx
Note: If you are using Windows Vista or 7, make sure to run the command: regsvr32 "%Systemroot%\System32\comdlg32.ocx" as and administrator.
You will see the following message: DllRegisterServer in C:\WINDOWS\System32\comdlg32.ocx succeeded.
You've re-registered the DLL!!!
Also, run (as an administrator) sfc /scannow in a command prompt.
I would also scan for spyware/viruses too.