Urlmon.dll is a system dll that's loaded at startup by Explorer (the system shell, not internet explorer). It may be damaged. You have to fish for an urlmon.dll and replace it via bootcd or Recovery Console and see if everything is fine.
- Find a correct urlmon.dll (if your other system is a Windows XP Service Pack 2 up to date take it there, or look for a backup copy in the same computer. An older version may work well too, but if you can, go for the latest possible one). Save it to a floppy (if you have it).
- Boot Recovery Console (or prepare an UBCD 4 Windows and boot it).
- Enter the c:\windows\system32 directory
- Rename urlmon.dll in urlmon.bak
- Copy the new urlmon.dll in the same folder.
Don't delete the original one, rename it in case you need it later.
In case you didn't do a chkdsk, do it as soon as possible.
I had to fix a PC this afternoon: it crashed during Windows Update, and the file ntfs.sys was gone. I didn't even have to repair it: a chkdsk from recovery console fixed the problem perfectly without having to replace the dll.
Otherwise, if applicable, reinstall, but I wouldn't do it. I seldom do it. Unless I have a badly mangled hard disk, when I am able to trace back the problem I ALWAYS investigate it and try to solve it. It's much better than reinstalling for a number of reasons, the most important being a far better understanding of the problem, with all the added advantages.
If and when the problem is solved, check your hard disk for bad sectors. I use HDTune and HDDHealth, the second one because HDTune gives some false positives on SMART parameters when default data values are too close to the threshold value (for instance 100 and 97). HDDHealth does far less than HDTune, but I use it to double check SMART results when I think I may have a false positive. Remember to check the Event registry for added clues.
