Please forget about above post#3.

Thank you for your quick response!

Edit\\
OS: WinXP Home. (Operating System installed in a PC having the .mht file error)
Web Browser: IE7. (Default web browser used in the PC)

(I really apologize for: no provision of the OS and browser info at above post #1; I could not edit the post within 24 hours* of posting the first post due to my work ----- *About a month ago, General Keebler had set 24 hour time limit on edition of a post in this forum (here) , to help combat spam here)

The .xls file's "Open…" option was already selected when I saw the option on Tuesday (12/21/2006). The option has been selected since the Tuesday.

A MHT file’s "Open…" option cannt be selected currently since it is grayed. (To see the grayed option, Please check an image attached to post#3 above).

I' unable to open a .mht file with firefox too ; I get "Firefox doesn't know how to open this address, because the protocol (mhtml) isn't associated with any program" error upon the openig. (To see the error, Please check an image attached to this post)

Thank you Kentigern and all!