Start with the basics, in order to troubleshoot the XPCOM.DLL error:
Close Netscape
Open Windows Explorer
Browse to your profile directory
C:\Documents and Settings\user_name\Application Data\Mozilla\Profiles\profilename\xxxxxxxx.slt
C:\Windows\Application Data\Mozilla\Profiles\profilename\xxxxxxxx.slt
(Depending on your OS)
Delete the following files:
history.dat
history.mab
panacea.dat
Cache folder
Chrome folder
All msf files in the Mail and News Folders
Restart Netscape
If still no joy:
Create a new Profile.
Another approach is to install xpcom.xpi. The file could have become corrupt during the install of Netscape 7.