What OS do you have? If Win95, Win98, or WinMe (or any OS AND you're using FAT or FAT32 file system), you can boot to a floppy and check if you're missing files.
Otherwise, I have no idea what it is. mpagent.exe is a Windows Media player file - probably nothing to do with your error.
What was the last thing you did/installed/ran before this error?
Can you boot to SAFE mode? Hold down Shift while windows is loading (or just before it starts loading) to access the DOS-like menu to go into safe mode (there might be another way - ask around or search Google). If SAFE mode works, you might be able to figure things out from there.
-Nerseus