NS_alloc is a network service in Windows. If there's problems with it, you might have problems with internet as well.
No idea for the xpcom.dll, but according to the forums, it's an important file in Firefox. Without it, it will cause problems.
The recommendation is to reinstall Firefox.
http://www.ufaq.org/modules.php?name=Forums&file=viewtopic&p=21255To reinstall an older version of Firefox:
http://www.oldversion.com/program.php?n=firefox