The short answer is you probably can fix the MBR.
Does your system disk have valuable information on it you haven'et backed up, or as a worst case, could your reinstall the OS with the restore disks and start anew?
Since you don't have an installation DVD, and can't boot into the OS, to get to System Recovery, it is a problem.
Here is a link that Microsoft discusses how to use BootRec.exe to repair the MasterBootRecord. http://support.microsoft.com/kb/927392
Since it is fairly technical, and you have to download BootRec.exe and run it from the Cmd line, you might want to ask a friend that is familiar with command line computing to give you a hand.
Hope that's helpful.